01 - Heat a large skillet over medium heat. Add ground beef and cook, breaking it up with a spatula, until browned and nearly cooked through, about 5 minutes.
02 - Add the chopped onion and garlic to the skillet, cooking until onion is soft and translucent, about 4 minutes.
03 - Stir in Worcestershire sauce, smoked paprika, salt, and black pepper. Cook for 2 more minutes, then remove from heat.
04 - In a bowl, combine cheddar and mozzarella cheeses until evenly mixed.
05 - Lay out the bread slices. Spread softened butter on one side of each slice. For extra crispiness, spread a thin layer of mayonnaise on the opposite side.
06 - Place 4 bread slices, butter side down, on a clean surface. Top each with a generous spoonful of the beef mixture and a hefty handful of the cheese blend. Cover with remaining bread slices, butter side up.
07 - Heat a large skillet or griddle over medium heat. Grill the sandwiches in batches, pressing lightly, 3-4 minutes per side or until bread is golden brown and cheese is completely melted.
08 - Let sandwiches cool for 1 minute before slicing diagonally. Serve immediately while hot and melty.