01 - Spread butter evenly on one side of each sourdough slice.
02 - Lay two slices buttered side down on a clean work surface.
03 - Spread 1 to 1½ tablespoons of basil pesto on the unbuttered side of each slice.
04 - Arrange mozzarella slices over the pesto, adding provolone and cheddar if desired for deeper flavor.
05 - Top each with the remaining bread slices, buttered side facing up.
06 - Place a large nonstick skillet or griddle over medium-low heat and allow it to warm through.
07 - Cook each sandwich for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is deeply golden and the cheese has fully melted.
08 - Transfer sandwiches to a cutting board, let rest 1 to 2 minutes for the cheese to set, then slice diagonally and serve warm.