Make the Sloppy Joe Filling:
In a skillet over medium heat, cook ground beef and onion until beef is browned and onion is soft. Drain excess fat.
Stir in ketchup, Worcestershire sauce, mustard, salt, and pepper. Simmer for 5–7 minutes until thickened. Remove from heat.

Assemble Sandwiches:
Butter one side of each bread slice. Place 4 slices butter-side-down on a skillet or griddle over medium heat.
Top each with a slice of cheddar, a generous scoop of sloppy joe mixture, then another slice of cheddar. Top with the remaining bread slices, butter-side-up.

Cook:
Grill sandwiches for 3–4 minutes per side or until bread is golden brown and cheese is melted. Press gently with a spatula to compact.

Serve:
Cut in half and serve hot. Enjoy the perfect combo of sloppy joe goodness with melty grilled cheese!