Bring a large pot of salted water to a boil. Add noodles and cook according to package instructions, typically 3-5 minutes. Drain, rinse with cold water, and toss with sesame oil.

In a skillet, heat 1 tbsp vegetable oil over medium-high heat. Add ground beef, break apart, and cook until browned, about 5-7 minutes.

Remove beef, add more oil if needed, then sauté chopped onion for 2-3 minutes until translucent. Add minced garlic, grated ginger, and green onion whites; cook for 1 minute until fragrant.

Add sliced bell pepper and julienned carrots to the skillet, stir-fry for 2-3 minutes until tender but crisp.

In a bowl, whisk together soy sauce, oyster sauce, hoisin sauce, sesame oil, brown sugar, and red pepper flakes. Mix well.

Add cooked noodles, browned beef, and sautéed vegetables back to the skillet. Pour sauce over and toss to coat evenly. Stir-fry for an additional 2-3 minutes.

Remove from heat and serve in bowls, garnished with sesame seeds and chopped green onions.