Prepare the Beef: Slice the beef sirloin or flank steak thinly against the grain. Season lightly with salt and pepper if desired.

Heat the Oil: Heat 1 tablespoon of vegetable oil in a large pan or wok over medium-high heat. Add the sliced beef and stir-fry for 2-3 minutes, until browned but not fully cooked.

Cook the Vegetables: In the same pan, add the remaining 1 tablespoon of oil. Add the sliced red and green bell peppers, along with the onion, and stir-fry for 3-4 minutes until they start to soften.

Add Garlic and Ginger: Add the minced garlic and ginger to the vegetables. Stir-fry for another minute until fragrant.

Make the Sauce: In a small bowl, whisk together the soy sauce, honey, sriracha, and rice vinegar. Pour the sauce into the pan with the vegetables, stirring to coat evenly.

Thicken the Sauce: Stir the cornstarch mixture (cornstarch mixed with water) into the pan to thicken the sauce. Allow the sauce to cook for 1-2 minutes until it thickens slightly.

Combine the Beef and Vegetables: Return the cooked beef to the pan, tossing everything together. Stir-fry for an additional 2-3 minutes until the beef is cooked through and coated with the sauce.

Garnish and Serve: Remove the pan from the heat and garnish with sesame seeds and chopped green onions, if desired.