Prepare the Bread: Toast all 12 slices of bread until golden brown.
Assemble the First Layer: Spread a thin layer of mayonnaise on one slice of toast. Add a slice of cheese, 2 slices of turkey or chicken, a tomato slice, and a lettuce leaf.
Add the Second Layer: Place another slice of toast on top of the first layer. Spread mayonnaise on this slice and repeat the same filling: cheese, turkey or chicken, tomato, and lettuce.
Top the Sandwich: Cover the second layer with the third slice of toast.
Secure and Slice: Insert toothpicks at the center of each sandwich quarter to hold the layers in place. Use a sharp knife to cut the sandwich diagonally into four triangles.
Serve: Arrange the sandwich triangles on a platter and serve immediately with your choice of side (fries, chips, or salad).