First up, heat your skillet over medium‑high. Toss in the lean ground beef, then immediately add the minced garlic and ginger. I let it sizzle until the beef is nicely browned and everything smells amazing, breaking it up as it cooks. That caramelization gives so much character to the dish.
While the beef is browning, whisk together the soy sauce, brown sugar, hoisin, and a splash of water. I like to taste it before adding—this is where you tweak if it's too salty or sweet—remember earlier tips on balancing! Pour it over the beef and stir well. Let it heat through so all the flavors blend, about 2‑3 minutes .
Now mix the cornstarch into the remaining water to make a smooth slurry. Pour it into the skillet and stir continuously. It only takes a minute or two to thicken, and you'll know it's ready when the sauce coats the beef in a glossy clingy layer.
Turn the heat to low and sprinkle in the red pepper flakes and black pepper. Stir in the green onions at the very end to retain their bright color and crisp bite. Give it a final taste—if it needs a tiny splash more soy or heat, go for it.
Serve this Mongolian ground beef piping hot over rice, noodles, cauliflower rice, or inside lettuce wraps. For leftovers, I let it cool to room temperature before refrigerating. It keeps well for 3‑4 days in an airtight container. Reheat gently over medium heat, adding a bit of water or broth to loosen up the sauce. You can freeze it (without rice) for up to 2 months—just thaw overnight and reheat.