In a large skillet over medium heat, brown and season with kosher salt, the ground beef until fully cooked. Drain any excess fat.
Add the diced onion, green bell pepper, and minced garlic to the skillet with the beef. Cook for 3-4 minutes, or until the vegetables have softened.
In a small bowl, whisk together the ketchup, tomato paste, Worcestershire sauce, and Revolution Barbecue Kansas City Style BBQ Rub.
Add the BBQ sauce mixture to the skillet with the beef and vegetables. Stir to combine and cook for an additional 5 minutes, or until the mixture has thickened slightly.
Heat a griddle or non-stick skillet over medium-high heat. Spread softened butter on one side of each slice of sourdough bread.
Place the bread butter-side down on the griddle or skillet. Top each slice of bread with a slice of cheddar cheese.
Spoon the BBQ beef mixture onto one slice of bread. Top with the other slice of bread and press down gently.
Cook the sandwich on the griddle or skillet until the bread is golden brown and the cheese is melted, about 2-3 minutes per side.
Serve hot and enjoy your Kansas City Style BBQ Sloppy Joe Sandwich!