In a medium mixing bowl, combine the thinly sliced beef sirloin with soy sauce, sesame oil, minced garlic, and grated ginger. Mix well to ensure the beef is evenly coated. Allow the mixture to marinate for approximately 10 minutes at room temperature to enhance the flavor.
While the beef is marinating, prepare your vegetables: slice the bell peppers into strips, separate the broccoli into small florets, and trim both ends of the snap peas.
Heat a large skillet or wok over medium-high heat and add a splash of vegetable oil (such as canola or peanut oil) to the pan—about a tablespoon should suffice. Once the oil is shimmering and hot, add the marinated beef in a single layer. Sear the beef for about 2-3 minutes without stirring, allowing it to brown, before stirring occasionally to cook through.
Once the beef is cooked and nicely browned, carefully remove it from the skillet and set it aside on a plate, ensuring to leave behind any excess juices in the pan. In the same skillet, add the sliced bell peppers, broccoli florets, and snap peas. Stir-fry the vegetables for approximately 3-4 minutes until they are vibrant in color and slightly tender yet still crisp.
Return the cooked beef to the skillet with the veggies, mixing everything together thoroughly. Continue to stir-fry for an additional 2 minutes to allow the flavors to meld and warm the beef through.
Season the dish generously with salt and freshly ground pepper to taste. Remove from heat and garnish the stir fry with finely chopped green onions and a sprinkle of lightly toasted sesame seeds, if using.
Notes
Serve hot over steamed rice or noodles for a complete meal.