01 - Spread butter evenly on one side of each sourdough bread slice.
02 - Lay two bread slices butter-side down on a clean work surface.
03 - Spread 1 to 1.5 tablespoons of basil pesto on the unbuttered side of each slice.
04 - Arrange mozzarella slices on top of the pesto, adding provolone and cheddar if desired for a more complex flavor profile.
05 - Place the remaining bread slices on top, buttered side facing outward, to complete each sandwich.
06 - Set a large nonstick skillet or griddle over medium-low heat and allow it to preheat.
07 - Cook each sandwich for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is golden brown and the cheese is fully melted.
08 - Transfer sandwiches from the pan and let rest for 1 to 2 minutes. Slice diagonally and serve warm.