Thinly cut flank steak against the grain, then dice.
In a medium bowl, sprinkle beef with salt and pepper, and toss with cornstarch until well combined.
To the bottom of the slow cooker, add oil. Add beef, garlic, ginger, brown sugar, sesame oil, soy sauce, and water and mix until well combined.
Turn the slow cooker on 4-5 hours on low, or 2½ hours on high.
Stir before serving. Serve over rice. Garnish with green onions and sesame seeds.
Notes
Pro Tip: Place steak in the freezer for about 20 minutes to make it easy to slice.Pro Tip: Cut across the grain in the meat to get the most tender pieces.
Keep leftover Mongolian beef in a covered container in the refrigerator for up to 4 days. Freeze in individual portions in zip top bags for up to 4 months.