Prepare the Sandwich: Layer American cheese, mozzarella, and a bit of grier cheese between two slices of sourdough bread. Spread a little bit of butter on the outside of the bread.
Cook the Sandwich: Press the sandwich together and cook in a pan over very low heat. Press down on the sandwich lightly while it cooks to ensure even melting of the cheese.
Serve: Serve the grilled cheese sandwich once the cheese is ooey gooey and the bread is toasted to your liking.