In a large skillet over medium heat, cook the ground beef until browned, breaking it apart with a spatula.
Add the diced onion, green bell pepper, and minced garlic to the skillet. Cook until the vegetables are softened, about 5 minutes.
Stir in the tomato sauce, Worcestershire sauce, brown sugar, and mustard. Season with salt and pepper to taste.
Reduce the heat to low and let the mixture simmer for about 10 minutes, allowing the flavors to meld together.
Toast the hamburger buns until golden and crispy.
Spoon the beef mixture onto the bottom half of each bun, then top with a generous amount of crispy fried onions.

Place the top half of the bun on each sandwich and serve immediately.
Variations & Tips
For a spicier version, add a pinch of cayenne pepper or a few dashes of hot sauce to the beef mixture. You can also substitute ground turkey or chicken for a lighter option. For a vegetarian version, use lentils or a meat substitute instead of ground beef. Add some shredded cheese on top of the beef mixture before adding the crispy onions for an extra layer of flavor. If you prefer a sweeter sauce, increase the amount of brown sugar to taste.