Place bread on work surface and spread the outsides of both slices with either butter or mayonnaise, making sure to cover the surfaces. Spread the interiors with either butter, mustard, or fruit preserves. Top one slice of bread with 2 slices of cheese and a combination of your favorite fillings. Sandwich together with second slice of bread.
Preheat a well-seasoned cast-iron skillet over medium heat. Add the assembled sandwich. Once the cheese starts to melt and the bread on the bottom is golden brown, flip the sandwich; avoid pressing with the spatula to flatten. Grill until cheese has fully melted and the other side is golden brown.