Preheat a non-stick skillet or griddle over medium heat.
Butter one side of each slice of sourdough bread with the softened butter. If using mayonnaise, spread it on the opposite side of one slice of bread.
Place one slice of bread, buttered side down, onto the skillet.
Layer one slice of cheddar cheese, followed by a slice of provolone cheese, and then the slice of mozzarella cheese on the bread in the skillet.
Top with the second slice of cheddar cheese and then a slice of provolone cheese.
Place the second slice of sourdough bread on top, buttered side up.
Cook for 4-5 minutes on one side or until the bread is golden brown and the cheese starts to melt.
Carefully flip the sandwich and cook for an additional 4-5 minutes or unIl the other side is golden brown and the cheese is fully melted.
Remove from the skillet and let it sit for a minute before slicing.
Cut the sandwich in half and serve immediately.
Notes
Cooking Temperature: The cooking time is based on a medium heat setting. Monitor the heat closely during testing, as cooking on too high of a temperature could cause the bread to brown too quickly
without melting the cheese.