Name: Bún song thằn xào bò — Bình Định mung-bean noodles with beef
Prep time: 25 min
Cook time: 18 min
Total time: 53 min
Cuisine: Vietnam
Ingredients: 250 g dried bún song thằn (mung-bean-starch noodles), broken into manageable lengths after soaking if sold as a sheet; 300 g top-sirloin steak, trimmed and sliced thinly across the grain; 150 g mustard greens, washed, with stems and leaves sliced separately; 100 g carrot, peeled and cut into fine matchsticks; 100 g onion, thinly sliced; 10 g garlic, finely minced; 20 g scallions, thinly sliced, whites and greens separated; 30 ml neutral oil, divided; 30 ml fish sauce; 150 ml warm water; 5 g sugar; 1 g ground black pepper, divided; 15 ml fresh lime juice

Wide wooden-table view with the plate of beef and mung-bean noodles on the right and open table space to the left. — A wide view of Bình Định bún song thằn xào bò with beef and greens.

Yield 4 portions

Cook 18 minutes

Passive 10 minutes for soaking

Total 53 minutes

Calories per serving about 421 kcal

Ingredients ↓

Overview

Bún song thằn xào bò — Bình Định mung-bean noodles with beef

Bún song thằn is a specialty mung-bean-starch noodle associated with An Thái village in Bình Định. The dried strands turn translucent and springy when cooked, and local accounts describe both stir-fried versions and noodle soups. Beef and greens are among the documented stir-fry combinations. This measured home recipe uses thinly sliced beef, mustard greens, carrot and onion; it is one practical version, not a single fixed traditional formula.

The noodles need a shorter soak than many rice noodles. Leave them in cool water only until they bend without snapping, then let the wok finish softening them. That keeps their distinctive chew and helps the strands stay separate. Fish sauce, garlic and lime season the dish without masking the bean aroma.

Practical

Tips for successFollow the noodle packet if it gives a different soak time; otherwise begin checking after 5 minutes and drain as soon as the strands are flexible. Slice beef across the grain and cook it in uncrowded batches so it sears instead of releasing water. Add the seasoning liquid in stages. The noodles should finish translucent and springy, with no hard centre or pool of sauce.

Ingredients

Noodles and beef

250 g dried bún song thằn (mung-bean-starch noodles), broken into manageable lengths after soaking if sold as a sheet
300 g top-sirloin steak, trimmed and sliced thinly across the grain

Vegetables and aromatics

150 g mustard greens, washed, with stems and leaves sliced separately
100 g carrot, peeled and cut into fine matchsticks
100 g onion, thinly sliced
10 g garlic, finely minced
20 g scallions, thinly sliced, whites and greens separated

Seasoning and cooking liquid

30 ml neutral oil, divided
30 ml fish sauce
150 ml warm water
5 g sugar
1 g ground black pepper, divided
15 ml fresh lime juice

Method

Slice the beef across the grain and keep it refrigerated while you prepare the vegetables. Whisk the fish sauce, warm water, sugar and half the pepper in a small bowl. Set out a clean plate and clean tongs for the cooked beef.
Cover the dried bún song thằn with cool water and soak just until flexible, usually 5–7 minutes. If the strands are still brittle, soak for 2 minutes longer and check again. Drain very well; if the noodles came as a square sheet, cut or break them into manageable lengths. Do not soak until fully soft.
Heat a large wok over high heat. Add 10 ml oil and sear half the beef in a single layer, turning once. Cook the thickest strips to 71°C / 160°F, checking with a food thermometer, then transfer them to the clean plate with clean tongs. Repeat with another 10 ml oil and the remaining beef. Do not crowd the wok.
Add the remaining 10 ml oil to the wok. Stir-fry the garlic for about 15 seconds, then add the onion and carrot and toss for 2 minutes. Add the mustard-green stems and cook for 1 minute; add the leaves and scallion whites and toss until just wilted.
Add the drained noodles and pour in about two-thirds of the seasoning liquid. Toss continuously for 2 minutes, lifting and separating the strands with tongs. Add the remaining liquid in stages as needed; stop when the noodles are translucent, springy and hot, with no hard centre and no puddle in the wok.
Return the cooked beef and its juices to the wok. Toss for 1 minute until evenly hot. If the pan dries before the noodles soften, add warm water 15 ml at a time rather than flooding the noodles.
Turn off the heat. Add the lime juice, remaining pepper and scallion greens, toss once more and serve immediately. Refrigerate leftovers within 2 hours in shallow containers.

Practical

Practical guidance

Keep raw beef, its juices and the ready-to-eat vegetables separate. Wash your hands, board and knife after handling the beef, and use clean tongs when it returns to the wok. Check the noodle packet for added wheat or soy and for cross-contact information; the traditional noodle is made from mung-bean starch, but packaged products vary. Refrigerate leftovers within 2 hours in shallow containers at or below 4°C / 40°F. Use within 3–4 days and reheat to 74°C / 165°F throughout. The noodles are best just after cooking, while their strands are still springy.

Troubleshooting

The noodles clump: They may have soaked too long or drained poorly. Separate them with tongs and add the seasoning in small pours over high heat. The noodles are still firm: Add warm water 15 ml at a time and toss for another minute until the centre is tender. The wok turns watery: Cook the vegetables and beef in batches, drain the noodles well and let excess liquid evaporate before adding more seasoning.

Frequently asked questions

What are bún song thằn noodles made from? They are traditionally made from mung-bean starch rather than rice flour. Dried products may be sold as strands or in a compact sheet. Can I use ordinary glass noodles? Mung-bean glass noodles can work as a practical substitute, but the dish will no longer use the Bình Định specialty noodle named in the recipe. Can I make this without beef? Yes. A vegetable-only version is possible, but it is a different adaptation; cook any replacement meat to its safe temperature before mixing it back into the noodles.

Nutrition per serving (estimate)

Serving size1 of 4 portions
Calories421 kcal
Carbohydrate63 g
Protein19 g
Fat10 g
Fibre3 g
Sodium720 mg

Allergens and food safety

Fish sauce contains fish. Check the noodle and sauce packaging for wheat, soy or cross-contact with other allergens. Keep raw beef separate from vegetables and cooked food. Cook the thickest beef strip to 71°C / 160°F as a conservative endpoint. Refrigerate leftovers within 2 hours and reheat to 74°C / 165°F throughout.

To finish

Serve hot with lime wedges and extra sliced scallions.
Put any leftovers into shallow containers and chill promptly.

Bún song thằn xào bò — Bình Định mung-bean noodles with beef

Practical

Ingredients

Noodles and beef

Vegetables and aromatics

Seasoning and cooking liquid

Method

Practical

Practical guidance

Troubleshooting

Frequently asked questions

Nutrition per serving (estimate)

Allergens and food safety

To finish

Vietnamese National Dishes

Gỏi gà bắp cải Nam Bộ — Southern Vietnamese chicken and cabbage salad

Bánh hỏi heo quay — Vietnamese woven rice noodles with roast pork

Xôi vùng cao — Lào Cai-inspired five-colour sticky rice