Marinate the Beef:
In a mixing bowl, toss thinly sliced beef sirloin with 2 tablespoons soy sauce and 1 tablespoon cornstarch until well coated. Set aside and allow to marinate for 10–15 minutes while preparing other ingredients.
Cook the Noodles:
Bring a large pot of water to a boil and cook the noodles according to package directions until al dente. Drain, rinse briefly under cold water, and set aside.
Sear the Beef:
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add marinated beef and stir-fry for 4–5 minutes or until browned and just cooked through. Remove from skillet and set aside.
Stir-Fry the Vegetables:
In the same skillet, add the remaining 1 tablespoon of oil. Add sliced onion and garlic, cooking for about 1 minute until fragrant. Add carrots, bell peppers, broccoli, and snow peas. Stir-fry for 5–7 minutes or until vegetables are tender-crisp. Add a splash of water and cover briefly to help steam the broccoli if needed.
