Heat a large skillet over medium heat. Add a splash of oil and toss in the chopped onions and bell peppers. Sauté for about 5 minutes, or until they're softened and fragrant.

Add the lean ground beef to the skillet, breaking it apart with a spatula. Cook it for 6-8 minutes until it’s nicely browned.

Stir in the minced garlic, along with salt and pepper, and cook for about 1 minute until the garlic becomes fragrant.

Pour in the tomato sauce, Sriracha, Worcestershire sauce, and brown sugar. Mix everything well until combined, then let this simmer on low heat for about 10 minutes.

Spoon the spicy filling generously onto each soft burger bun. Serve immediately.