Cook the lo mein noodles according to package instructions; drain and set aside.
Heat oil in a large skillet over medium-high heat.
Add minced garlic and stir-fry for about 30 seconds until fragrant.
Add sliced beef, season with salt and pepper, and cook until browned (about 3-4 minutes).
Add mixed vegetables and stir-fry for another 2-3 minutes.
Pour in soy sauce and oyster sauce, mixing well.
Add the cooked noodles to the skillet, tossing everything together.
Drizzle with sesame oil and mix well.
Stir in chopped green onions and adjust seasoning if necessary.
Serve hot and enjoy your homemade beef lo mein!