01 - In a medium mixing bowl, combine shredded cheddar, Gruyère, mozzarella, and fontina cheeses until evenly blended.
02 - Arrange the sourdough slices on a work surface. Spread a thin, even coating of softened butter on one side of each slice.
03 - Drizzle 1 teaspoon truffle oil over the unbuttered side of two bread slices.
04 - Distribute the cheese mixture evenly onto the two truffle-oiled sides.
05 - Sprinkle finely chopped chives and black pepper over the cheese if desired.
06 - Top with the remaining bread slices, ensuring the buttered sides face outward.
07 - Preheat a large skillet over medium-low heat. Place assembled sandwiches in the skillet and cook for 3 to 4 minutes per side, applying gentle pressure, until the bread is crisp and golden and the cheese is thoroughly melted.
08 - Transfer sandwiches to a cutting board, let them rest for 1 minute, then slice and serve while hot.