Heat a large skillet over medium-high heat. Add the ground beef and break it apart with a wooden spoon. Cook until the beef is no longer pink, about 5-7 minutes. If using higher fat beef, drain excess fat, leaving about 1 tablespoon in the pan.
Add the diced onion and bell pepper to the browned meat. Sauté for about 4-5 minutes until the vegetables soften. Add the minced garlic and cook for an additional 30 seconds until fragrant.
Stir in the ketchup, brown sugar, apple cider vinegar, Worcestershire sauce, mustard, tomato paste, smoked paprika, and red pepper flakes if using. Mix thoroughly to combine all ingredients.
Reduce heat to medium-low and simmer the mixture for 10-12 minutes, stirring occasionally. The sauce should thicken slightly and the flavors will meld together. Season with salt and pepper to taste.
Toast your hamburger buns for optimal texture contrast. Using a slotted spoon, generously spoon the sloppy joe mixture onto the bottom half of each bun, then cap with the top half. Serve immediately.