In a bowl, combine the sliced sirloin steak, soy sauce, and sesame oil. Marinate for at least 15 minutes.
Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the marinated steak to the skillet and cook for about 3-4 minutes, stirring frequently, until browned and cooked to your desired doneness. Remove the steak from the skillet and set aside.
In the same skillet, add the broccoli, bell peppers, and zucchini. Stir-fry for about 5-6 minutes until the vegetables are tender-crisp.
Return the steak to the skillet with the vegetables and stir to combine. Season with salt and pepper to taste.
Serve the steak and vegetable mixture over cooked rice. Garnish with sliced green onions and sesame seeds.