Preheat the oven to 450 degrees F.
In a small bowl, combine the softened butter, Parmesan cheese, and chopped parsley.
Take about 1 tablespoon of the butter mixture and spread it evenly over one side of each slice of bread.
Place 4 slices of bread, butter-side down, onto a baking sheet. Add a slice of Gruyère to each slice of bread. Divide the shredded cheddar cheese between the four slices. Place the 4 remaining slices of bread, butter-side up, on top of the slices with cheese.
Place the baking sheet in the preheated oven and bake for 5 minutes.
Open the oven and quickly flip the sandwiches over, then bake for another 5 minutes, or until the bread is a delicious golden brown, and the cheese is fully melted.