For the garlic mayonnaise: mix all ingredients together and refrigerate.
For the sandwiches: On each slice of bread, spread butter on one side and garlic mayo on the opposite.
On one slice, top with 2 pieces of American cheese, chicken tender, bacon, spinach, and 2 more pieces of American cheese. Top with another slice of bread, butter side out
In a panini maker or on a griddle or frying pan, cook grilled cheese until browned on both sides and cheese has melted. Cut in half and serve.