01 - Spread butter evenly on one side of each sourdough bread slice.
02 - Lay two slices buttered side down on a clean work surface.
03 - Spread 1 to 1.5 tablespoons of basil pesto evenly across the unbuttered side of each base slice.
04 - Arrange mozzarella slices over the pesto, adding provolone and cheddar if desired for a more complex flavor profile.
05 - Place the remaining bread slices on top with the buttered side facing outward to complete each sandwich.
06 - Heat a large nonstick skillet or griddle over medium-low heat until evenly warmed.
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 - Remove from the pan and let rest for 1 to 2 minutes before slicing diagonally. Serve warm.