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.