Heat a large skillet over medium heat. Add the ground beef and cook for about 5–7 minutes until browned and fully cooked.

Add diced onions and green bell peppers to the skillet with the beef. Sauté for 3–4 minutes until the vegetables soften and become fragrant.

Stir in the tomato sauce, ketchup, Worcestershire sauce, and mustard to the mixture. Allow the filling to simmer on low heat for about 5 minutes, stirring occasionally until it thickens.

Butter one side of each slice of bread generously. Place a spoonful of the prepared filling on the unbuttered side of half the slices, followed by a slice of cheddar cheese. Top with another slice of bread, butter side out.

In the same skillet, over medium-low heat, place the assembled sandwiches and cook for about 3–5 minutes per side until both sides are golden brown and the cheese is melted.

Once grilled, remove the sandwiches from the skillet and serve them hot, garnished with freshly chopped parsley or chives.