Slow-cooker Mongolian beef: tender slices, glossy sauce

This slow-cooker Mongolian beef gives you tender flank steak, a balanced sweet-savory sauce, and a glossy finish without complicated prep. You will cook the beef gently in a measured soy-brown sugar sauce, then thicken the cooking liquid separately so it clings to every slice instead of staying watery.

Plan on about 20 minutes of preparation and 4 to 5 hours on low or 2 to 3 hours on high. Keep the beef thinly sliced and check it early, because flank steak can turn dry if it stays in the cooker well past tender.

Ingredients for slow-cooker Mongolian beef (6 servings)

Yield: 6 servings. Prep time: 20 minutes. Cook time: 4 to 5 hours on low or 2 to 3 hours on high.

• 2 pounds flank steak, trimmed
• 1/2 cup low-sodium soy sauce
• 1/2 cup water or unsalted beef broth
• 1/3 cup packed light brown sugar
• 1/4 cup hoisin sauce
• 2 tablespoons unseasoned rice vinegar
• 1 tablespoon toasted sesame oil
• 4 garlic cloves, finely minced
• 1 tablespoon finely grated fresh ginger
• 1/2 teaspoon black pepper
• 1/2 teaspoon red pepper flakes, optional
• 2 tablespoons cornstarch
• 3 tablespoons cold water, for the slurry
• 3 scallions, thinly sliced
• 1 tablespoon sesame seeds
• 6 cups cooked white or brown rice, for serving
• 4 cups steamed broccoli or green beans, for serving

For the most tender result, use flank steak or a similarly lean, quick-cooking cut such as top sirloin. Put the steak in the freezer for 20 to 30 minutes before slicing; the firmer texture makes thin, even slices easier.

Look for the long muscle fibers running across the steak. Slice across the grain, cutting perpendicular to those fibers, into strips about 1/8 inch thick. If you slice with the grain, the cooked beef will feel stringy and chewy even when the sauce is right. Discard any large pieces of connective tissue, but do not cut the strips much thicker than 1/4 inch because they will need more time to tenderize.

Build the sauce for Crock-Pot Mongolian beef

1. Whisk the soy sauce, water or broth, brown sugar, hoisin sauce, rice vinegar, sesame oil, garlic, ginger, black pepper, and red pepper flakes in a bowl until the sugar begins to dissolve.
2. Place the sliced beef in the slow cooker and pour the sauce over it. Toss with tongs until every piece is coated, then spread the beef into an even layer.
3. Cover the slow cooker. Do not add the cornstarch yet; it can settle or lose its thickening power during several hours of cooking.

The sauce is intentionally concentrated because a slow cooker releases very little moisture. The vinegar keeps the brown sugar from tasting heavy, while the hoisin adds depth and helps the finished sauce coat the meat. If you only have regular soy sauce, replace 2 tablespoons of it with 2 tablespoons of water to keep the dish from becoming too salty.

Cook Mongolian beef in a slow cooker: low and high options

Cook on low for 4 to 5 hours or on high for 2 to 3 hours. Begin checking about 30 minutes before the shorter time ends. Thin slices are ready when they bend easily and feel tender when pressed with tongs. If they still resist or feel rubbery, cover the cooker and continue for 20 to 30 minutes before checking again.

Keep the lid closed during cooking so the temperature stays steady and the sauce does not lose moisture unevenly. Avoid holding the beef on the warm setting for several hours after it becomes tender; that extended heat can make lean flank steak dry. The sauce may look thinner than you expect at this stage. The final simmer and cornstarch slurry will create the proper consistency.

For a 6-quart cooker, the beef should sit mostly in the sauce rather than being packed into a tall mound. If you double the recipe, use a larger cooker and expect the cooking time to increase slightly. Do not start with frozen beef, which can cook unevenly and release excess water into the sauce.

Finish a slow-cooker Mongolian beef recipe with a cornstarch slurry

1. Use tongs to transfer the cooked beef to a serving bowl, leaving the cooking liquid in the slow cooker. Pour about 1 1/2 to 2 cups of the liquid into a small saucepan. If it contains visible fat, skim it off.
2. Bring the liquid to a steady simmer over medium-high heat and cook for 3 to 5 minutes. This concentrates the flavor and removes some excess water.
3. Whisk the cornstarch with the cold water in a small bowl until completely smooth. Whisk the slurry again, then pour it into the simmering sauce while stirring constantly.
4. Simmer for 60 to 90 seconds, stirring, until the sauce turns shiny and thick enough to coat a spoon. Do not boil it for several minutes after it thickens, or the texture can become pasty.
5. Pour the sauce over the beef and toss gently. If it is too thick, stir in 1 tablespoon of water at a time. If it is still thin, simmer it for another minute before adding more slurry.

Thickening the sauce in a saucepan gives you better control than adding cornstarch directly to the cooker. The finished sauce should cling to the beef in a smooth layer. If it runs quickly across the plate, reduce it a little longer; if it sits in a stiff mound, loosen it with a splash of water or broth.

Serve a Crock-Pot Mongolian beef recipe with rice and greens

Spoon the beef and sauce over hot rice, arranging steamed broccoli or green beans alongside it. Finish with the sliced scallions and sesame seeds. The rice absorbs the extra sauce, while the greens provide a fresh, lightly crisp contrast to the tender beef.

For the best texture, serve the beef as soon as you coat it with the thickened sauce. Add the scallions and sesame seeds at the table if you want the garnish to stay especially bright and crisp.