Preparation Position rimmed baking sheet on a rack in middle of oven and preheat to 450°F. Spread about 1 1/2 tsp. mayonnaise on one side of each slice of bread. Arrange 6 slices, mayo side down, on second inverted rimmed baking sheet. Mound 1/2 cup cheese in the center of each slice, season with salt and pepper, and top each with remaining bread slices, mayo side up. Using oven mitts, carefully remove preheated baking sheet from oven and place, rimmed side up, on top of sandwiches, then, holding both rimmed baking sheets together, return to oven. Bake until bread is browned and cheese is melted, about 15 minutes. Carefully remove baking sheets from oven and transfer sandwiches to a cutting board. Cut into pieces and serve.
Preparation Position rimmed baking sheet on a rack in middle of oven and preheat to 450°F. Spread about 1 1/2 tsp. mayonnaise on one side of each slice of bread. Arrange 6 slices, mayo side down, on second inverted rimmed baking sheet. Mound 1/2 cup cheese in the center of each slice, season with salt and pepper, and top each with remaining bread slices, mayo side up. Using oven mitts, carefully remove preheated baking sheet from oven and place, rimmed side up, on top of sandwiches, then, holding both rimmed baking sheets together, return to oven. Bake until bread is browned and cheese is melted, about 15 minutes. Carefully remove baking sheets from oven and transfer sandwiches to a cutting board. Cut into pieces and serve.