Cook the sweet potato starch noodles according to the package instructions, usually by boiling for about 7-10 minutes.
Drain and rinse with cold water to stop cooking. Cut the noodles into shorter lengths if desired.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced beef and cook until browned and cooked through, about 3-4 minutes.
Remove the beef from the skillet and set aside.
In the same skillet, add another tablespoon of vegetable oil.
Add the sliced onion and minced garlic, and sauté until fragrant, about 1 minute.
Add the julienned carrot, bell pepper, and mushrooms. Stir-fry for about 3-4 minutes until the vegetables are tender but still crisp.
Add the spinach and cook until wilted, about 1-2 minutes.
Return the cooked beef to the skillet with the vegetables.
Add the cooked noodles to the skillet.
In a small bowl, mix the soy sauce, oyster sauce (if using), sugar, and sesame oil.
Pour the sauce mixture over the beef and vegetable mixture in the skillet.
Stir well to coat everything evenly with the sauce.
Sprinkle with sesame seeds and chopped green onions.
Adjust seasoning with salt and pepper to taste.
Enjoy your meal!