Preheat a large pan over low-medium heat (alternatively you can use a panini style press or George-Foreman style grill). Grease the pan or panini grill with non-stick spray.
Spread mayo on both slices of bread followed by a slice of cheese on each. On one piece of bread, place a slice of cheese layered with spring mix, turkey, tomatoes seasoned with salt and pepper, another slice of cheese and top with the other bread slice.
Place the sandwich in the pan covered with a lid or on the panini grill and cook until the bread is golden brown and the cheese has melted. (If using a pan, flip the sandwich over and repeat for the other side, you may need to grease the pan with more non-stick spray.)