Place the lean ground beef, bell peppers, onion, garlic, flour, brown sugar, salt, and pepper in a heavy pot. Optional to add the red pepper flakes.
Place pot over medium-high heat. Break up the meat and stir as it heats. Cook and stir until most of the moisture has evaporated. The meat mixture should be crumbly and brown after 15 minutes. Stir in beef broth, ketchup, mustard, and Worcestershire sauce. Mix together and simmer for 45 minutes. Add ¾ cup of the cheese to the dip, and gently stir.
Preheat oven to broil.
Transfer the dip to a 9x13 pan (spray with cooking spray). Top with remaining cheese.
Place pan under broiler; broil until cheese is browned, and mixture is hot and bubbly, about 5 minutes.
Sprinkle finely chopped fresh parsley on top.
Serve with toasted crostini slices, and large Frito chips (for dipping).