Cook the noodles according to the package instructions. Drain and set them aside. Toss them with 1 tablespoon of sesame oil to keep them from sticking.
Prepare the Mongolian Ground Beef:
Heat a large skillet or wok over medium-high heat. Add the ground beef and cook, breaking it up with a spatula, until browned and fully cooked (about 5-7 minutes). Drain any excess fat from the beef.
Add the chopped onion, garlic, and grated ginger to the skillet. Stir-fry for 2-3 minutes until the onion becomes translucent and fragrant.
Make the Mongolian Sauce:
In a small bowl, whisk together the soy sauce, hoisin sauce, brown sugar, rice vinegar, oyster sauce (if using), and red pepper flakes. If the sauce looks too thick, add 1-2 tablespoons of water or beef broth to thin it out to your preferred consistency.
Add the sauce mixture to the cooked ground beef and stir to combine. Let it simmer for 2-3 minutes, allowing the sauce to thicken slightly. If you’d like a thicker sauce, mix 1 tablespoon of cornstarch with 2 tablespoons of water, then stir it into the sauce, cooking for another minute.
Combine the Noodles and Beef:
Add the cooked noodles to the skillet with the beef and sauce mixture. Toss everything together, ensuring the noodles are well-coated with the sauce and beef. Cook for an additional 2-3 minutes to let the noodles absorb the flavors.
Garnish and Serve:
Garnish with chopped green onions, sesame seeds, and fresh cilantro if desired. Serve immediately.
