Spread the mayo on the outsides of the bread slices.
Place one slice of the prepared bread, mayo side down, in a hot skillet or on a griddle.
Working quickly, place 2 slices of cheese on the bread, followed by the cooked chicken patty, marinara, 2 more slices of cheese, spinach, remaining cheese & second piece of bread (mayo side up) - IN THIS ORDER.
If using a grill press, close the lid.
Increase the heat to medium on your griddle or cooktop & grill until golden brown & cheese is melted.
If cooking in a skillet, you will need to use a spatula to flip the sandwich to cook the second side, cooking until both sides are golden.