Marinate the flank steak in a mixture of soy sauce, oyster sauce, brown sugar, and cornstarch for 15-20 minutes.
Cook the rice noodles according to package instructions, usually around 4-6 minutes, then drain and rinse under cold water.
Heat vegetable oil in a skillet or wok over medium-high heat, add marinated beef, and cook for 3-4 minutes until browned.
Sauté minced garlic and ginger for 30 seconds, then add sliced red bell pepper and snap peas, stir-frying for 2-3 minutes.
Combine cooked beef with sautéed vegetables, pour marinade sauce over, and cook for an additional 2-3 minutes.
Add cooked rice noodles to the skillet, toss everything together gently until well combined.