Preheat oven to 500 degrees F (260 degrees C). Wrap garlic in aluminum foil.
Bake garlic in the preheated oven until dark brown and softened, about 30 minutes. Remove garlic from the foil and let cool, about 7 minutes.
Reduce oven temperature to 350 degrees F (175 degrees C). Arrange bread slices on a baking sheet.
Peel garlic into a bowl; mash with butter to form a chunky paste. Mix in mayonnaise. Spread garlic butter over the bread slices; top with tomato and Parmesan cheese.
Bake in the preheated oven until the bread has absorbed the garlic butter, about 5 minutes.
