Ingredients
Scale
The Beef
- 2 lbs (900g) flank steak or skirt steak
- ¼ cup (30g) cornstarch
The Sauce
- ½ cup (120ml) soy sauce
- ½ cup (110g) packed brown sugar
- ¼ cup (60ml) beef broth
- 5–6 garlic cloves, minced
- 1 tbsp fresh ginger root, finely grated
- 2 tbsp hoisin sauce (optional but recommended)
- ½–1 tsp red pepper flakes (to taste)
The Cornstarch Slurry (to finish)
- 2 tbsp cornstarch
- 3 tbsp cold water
The Finish
- 1 tsp toasted sesame oil — added after cooking
For Serving
- Steamed jasmine rice
- 4–5 green onions, thinly sliced
- 2 tbsp toasted sesame seeds
- Steamed bok choy or broccoli
Instructions
- Slice the beef. For easier slicing, place the flank steak in the freezer for 20 to 30 minutes until partially firm but not frozen solid. Slice against the grain — perpendicular to the visible muscle fibers — into strips approximately ¼ inch thick. If the strips are long, cut them in half crosswise.
- Coat in cornstarch. Place the sliced beef in a large zip-lock bag or bowl. Add the cornstarch and toss until every piece is evenly coated. Shake off any excess. The beef should have a light, even coating — not clumped.
- Make the sauce. In a medium bowl or measuring jug, whisk together the soy sauce, brown sugar, beef broth, minced garlic, grated ginger, hoisin sauce (if using), and red pepper flakes until the sugar is dissolved.
- Build the slow cooker. Place the cornstarch-coated beef in the slow cooker. Pour the sauce over the beef and stir once gently to ensure the beef is evenly distributed and coated.
- Cook. Set the slow cooker to LOW and cook for 4 to 5 hours, until the beef is tender and has absorbed the sauce. The sauce will be thinner than the final intended consistency — this is normal.
- Thicken the sauce. In a small bowl, mix the 2 tablespoons of cornstarch with 3 tablespoons of cold water until completely smooth. Stir the slurry into the slow cooker. Switch to HIGH and cook uncovered for 20 to 30 minutes, stirring once or twice, until the sauce has thickened to a glossy, coating consistency. Alternatively: remove the beef with a slotted spoon, pour the sauce into a small saucepan, bring to a boil, add the slurry, and reduce for 3 to 5 minutes until glossy.
- Finish. Stir in the toasted sesame oil immediately before serving.
- Serve. Spoon the Mongolian beef and its sauce over steamed jasmine rice. Scatter green onions and toasted sesame seeds generously over the top. Serve with steamed bok choy or broccoli alongside.
Notes
- Slice against the grain — always. The grain of flank steak is clearly visible as parallel lines running the length of the cut. Slicing perpendicular to those lines shortens the muscle fibers and produces tender, yielding strips. Slicing with the grain produces long, intact fibers that are tough and chewy regardless of how long they are cooked. Identify the grain direction before making the first cut.
- Partial freezing makes slicing easy. Twenty to thirty minutes in the freezer firms the flank steak just enough to allow clean, thin, consistent slices without the meat sliding away from the knife. Fully frozen beef is too hard to slice; partially frozen is ideal. This single step produces noticeably more uniform slices than cutting from room temperature.
- The cornstarch coating is not optional. It absorbs the sauce during the slow cook, thickens the braising liquid from the inside, and gives the sauce something to grip. Without it, the beef sits in a thin, pale sauce that does not cling or coat. With it, the finished sauce is darker, thicker, and fully integrated with the beef.
- Sesame oil goes in at the very end. Toasted sesame oil added to the slow cooker at the start of a four-hour cook loses its distinctive fragrance entirely — the volatile aromatic compounds evaporate completely. Added immediately before serving, it is the aromatic signature of the dish. Ten seconds. Never skip it.
- The sauce will be thin before thickening. This is expected and not a sign that anything has gone wrong. The slow cooker traps moisture. The thickening step — slurry or stovetop reduction — is what converts the braising liquid into the glossy restaurant sauce. Do not serve without this step.
- Green onions are a flavor component, not decoration. The raw, fresh sharpness of thinly sliced green onion cuts through the sweet-savory richness of the Mongolian beef sauce and balances the plate. A generous scatter — not a polite garnish — is correct.
- The dish is sweet by design. Half a cup of brown sugar in a Mongolian beef sauce is a lot of sugar, and first-time makers sometimes question it. The sweetness is the point — it is what makes Mongolian beef Mongolian beef rather than generic beef in soy sauce. The soy sauce’s salinity and the ginger’s spice balance it. If the finished dish seems too sweet, add a small splash of rice vinegar or an extra teaspoon of soy sauce to adjust.
- Prep Time: 15 minutes
- Cook Time: 4–5 hours (on LOW)
- Category: Dinner, Main Dish
- Method: Slow Cooking
- Cuisine: American, Asian
- Diet: Dairy-Free, Gluten-Free