Mongolian beef made in a slow cooker: flank steak in a tamari and brown sugar glaze, with broccoli and carrots timed to stay intact.
Fifteen minutes of prep, four to five hours of cooking, and two optional stages that make a real difference if you do them.
Why This Recipe Works
- Coating the beef in cornstarch before it goes in does two things at once — it gives the slices something to brown against in the optional sear, and it thickens the sauce gradually as it cooks.
- Tamari rather than soy sauce is what makes this gluten-free, and it is also slightly less salty and rounder, which suits a sauce carrying a third of a cup of brown sugar.
- A tablespoon of rice vinegar against that much sugar is small and essential. Without it the glaze reads as sweet rather than as sweet-and-savoury.
- The optional 30-to-60-second sear per side is genuinely brief. It is there for colour and flavour, not to cook the beef, which is why it is measured in seconds.
Three Additions, Three Times
The beef needs four to five hours. The carrots need about two. The broccoli needs one. Putting them all in at the start gives you tender beef and two grey vegetables.
Hour zero
Cornstarch-coated beef, and the whole sauce
Tamari, beef broth, brown sugar, rice vinegar, sesame oil, garlic, ginger and pepper flakes whisked together and poured over. The cornstarch already on the beef begins thickening the sauce from the first hour.
2.5 to 3 hours
2 medium carrots, thinly sliced
Halfway. Thinly sliced carrots need about two hours to be tender without collapsing, and adding them at the start would leave nothing but sweetness in the sauce.
The last hour
3 cups broccoli florets
One hour is already generous for broccoli in a slow cooker. The recipe is explicit about the last hour, and it is the addition most often got wrong.
The last 15 to 20 minutes
1 tbsp cornstarch slurried with 1 tbsp water, if you want it thicker
The second use of cornstarch, doing a different job from the first. The coating on the beef thickens gradually; this is a deliberate correction at the end.
Off the heat
Green onions
Stirred through at the very end so they keep their bite, with more reserved for the top.
Ingredient Notes
- Flank steak
- Two pounds, sliced thinly against the grain. Flank is lean and strongly grained, so the slicing direction matters more here than the cooking time.
- Cornstarch, in two places
- A quarter cup to coat the beef and a further tablespoon for the optional slurry. Arrowroot is the grain-free alternative for the coating.
- Gluten-free tamari
- Half a cup, low-sodium. Tamari is brewed without wheat where most soy sauce is not, which is the single change making this recipe gluten-free.
- Fresh ginger
- A tablespoon, minced. Fresh rather than ground — it is one of the two aromatics carrying the dish and dried ginger tastes nothing like it.
How to Make It
Slice and coat the beef
Pat the flank steak dry and slice thinly against the grain. Toss with 1/4 cup of cornstarch or arrowroot until lightly coated.
Sear, optionally
Warm 2 tablespoons of oil in a skillet over medium heat and briefly sear the beef in batches, 30 to 60 seconds per side, just to brown. Transfer to the slow cooker.
Make and add the sauce
Whisk the tamari, beef broth, brown sugar, rice vinegar, sesame oil, garlic, ginger and red pepper flakes, and pour over the beef.
Cook, adding vegetables on a timer
Low for 4 to 5 hours, or High for 2 to 3, until the beef is tender. Add the sliced carrots at the 2.5 to 3 hour mark and the broccoli in the last hour.
Thicken and finish
For a thicker glaze, stir in the slurry during the last 15 to 20 minutes, then stir in the green onions.
Serve
Over jasmine rice or cauliflower rice, garnished with extra scallions and sesame seeds.
Success Tips
- Against the grain, thinly. Everything else is recoverable and this is not.
- Keep the sear to seconds if you do it at all.
- Respect the vegetable timings — the carrots at halfway, the broccoli in the last hour.
- Decide about the slurry at the end, once you can see how thick the sauce already is.
Serving Ideas
Jasmine rice or cauliflower rice, with extra scallions and sesame seeds. Rice matters more than usual here because the sauce is a glaze rather than a gravy — there is not much of it and it needs something to cling to.
Storage and Reheating
Refrigerate
Refrigerate up to 4 days. The beef absorbs more of the glaze overnight and the broccoli softens further.
Freeze
Freeze up to 3 months, though the broccoli comes back limp. Freezing before the broccoli goes in and adding it fresh on reheating is the better route.
Reheat
Gently in a pan with a splash of water or broth to loosen the glaze. The microwave works and tends to overcook the vegetables further.
Frequently Asked Questions
My beef came out chewy.
Almost always the slicing rather than the cooking. Flank has long, coarse muscle fibres, and cutting along them leaves those fibres full length in every slice. Cut across them and each piece is short-fibred and tender at the same cooking time.
Is the sear worth doing?
It adds colour and depth for about five minutes of work, and it is optional because the dish is good without it. If you skip it, the cornstarch coating still thickens the sauce — you simply lose the browning a slow cooker cannot produce.
Can I use a different cut?
Sirloin or skirt both work sliced the same way. Avoid stewing cuts like chuck, which want much longer than four hours to become tender and would be underdone here, and avoid anything already tender enough to eat quickly — a braise is wasted on it.
