- Preheat oven to 220°C (425°F). Place 2 baking sheets in oven for 10 min. to heat up.
- Meanwhile, spread half the mayonnaise onto 6 bread slices (one side only). Place slices – mayo-side down – onto a sheet of parchment paper, then spread with mustard. Lay marble cheese slices on top, leaving ½-in. (1-cm) border to allow room for melting. Place remaining bread slices on top. Spread with remaining mayonnaise.
- Carefully remove hot baking sheets from oven. Lift the parchment paper and sandwiches onto one sheet. Cover sandwiches with a second sheet of parchment. Set the second hot baking sheet so its underside is directly on the parchment on top of sandwiches.
- Place the stack -- baking sheets, parchment, sandwiches -- into the oven. Bake 8 to 10 min. until bread is golden brown and cheese is melted. Cut each sandwich into thirds as “soldiers” for dunking into soup, if desired. Serve warm.
For a salty-sweet version, spread a little apricot or fig preserves onto the mustard before adding the sliced cheese.