Heat a large skillet or frying pan over medium heat. Add the ground beef and cook until browned, breaking it up into crumbles as it cooks.

Add the diced onion, green bell pepper (if using), and minced garlic to the pan. Cook for a few minutes until the vegetables are softened

In a bowl, mix together the tomato puree, brown sugar, salt, and pepper.

Pour the sauce mixture into the skillet with the ground beef and vegetables. Stir well to combine.

While the Sloppy Joe filling is simmering, lightly toast the hamburger buns if desired.