Start by slicing the beef sirloin against the grain into thin strips for tenderness. Julie the carrots and slice the bell pepper into thin rounds. Mince the garlic and have your sauces ready.

Heat a large skillet or wok over medium-high heat and add vegetable oil. Once shimmering, add the beef strips and stir-fry for 3 to 5 minutes until browned. Remove the beef from the pan and keep warm.

In the same pan, add more oil if necessary, then toss in broccoli, julienned carrots, and bell peppers. Stir-fry for 3 to 4 minutes until tender-crisp.

Return the cooked beef to the pan, stir in minced garlic, soy sauce, and oyster sauce. Mix well and cook for another minute.

In a bowl, mix cornstarch with water to create a slurry, then pour it into the pan while stirring. Cook for 1 to 2 minutes until the sauce thickens.

Add pre-cooked noodles, mix gently to coat with sauce, adjust seasoning, garnish with green onions, and serve hot.