01 -
Heat oven to 400°F. Line a large baking sheet with parchment paper.
02 -
Spread butter generously on one side of each bread slice, going to edges. Optionally sprinkle with Parmesan.
03 -
Place 4 slices butter-side down on sheet. Mix cheddar and Gruyère, divide between slices. Top with remaining bread butter-side up.
04 -
Bake on bottom rack 4-5 minutes until golden underneath and cheese melts.
05 -
Flip sandwiches and bake 2-3 minutes more. Let cool slightly before cutting.