Cook the beef: In a skillet over medium heat, add the ground beef and chopped onion. Break the meat apart with a wooden spoon and cook until the beef is no longer pink and onions soften, about 8–10 minutes.

Add sauce and seasonings: Stir in the tomato sauce (or ketchup), brown sugar, mustard, and Worcestershire sauce. Season with salt and pepper. Reduce heat to low and simmer gently for another 3–5 minutes, stirring occasionally, until the mixture thickens but remains moist.

Prepare the bread: While the beef simmers, spread a thin layer of softened butter on one side of each bread slice. This side will touch the skillet and crisp beautifully.

Build the sandwiches: Place four bread slices butter-side down on a clean cutting board. Sprinkle about half a cup of shredded cheddar over each. Then add a generous spoonful of the sloppy joe meat on top of the cheese. Top with another generous layer of cheese, then place the remaining bread slices on top (butter-side up).

Grill: Warm a clean skillet or non-stick pan over medium-low heat. Carefully transfer each sandwich to the skillet. Press gently with a spatula, and cook until the bottom bread slice is golden brown and the cheese begins to melt, about 3–4 minutes. Carefully flip the sandwich and cook the other side until golden and the cheese is fully melted.

Rest and serve: Remove sandwiches from the skillet, let them rest for a minute so the filling sets a little, then slice in half diagonally and serve hot.