Wash and slice the vegetables. Thinly slice the beef sirloin against the grain, slice the bell peppers into strips, cut the broccoli into small florets, trim the ends of the snap peas, mince the garlic and ginger, and chop the green onions.
In a medium bowl, combine the sliced beef, soy sauce, and cornstarch. Mix well and let marinate for 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry for 3-4 minutes until browned and cooked through. Remove from skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced bell peppers, broccoli florets, and snap peas. Stir-fry for 5 minutes until tender-crisp. Add minced garlic and ginger, stir-fry for an additional minute.
Return the cooked beef to the skillet with the vegetables. Pour in the sesame oil and toss everything together. Cook for another 2 minutes until heated through.
Garnish with chopped green onions and serve over rice or noodles.