In a small, microwave safe bowl, combine the cheese, sour cream, and Dijon mustard. Season to taste with salt and pepper.

Microwave the mixture for 30-45 seconds, just until warmed through and melting on the edges. It won’t be melted all the way. Stir to combined.

Lay the pieces of bread out and spread 1/4 tablespoon of butter on one side of each bread. Turn 4 slices over, and divide the cheese mixture between the 4 slices. Place another slice of bread on top, buttered side out, making 4 sandwiches.

Heat a skillet over medium heat. (This works best if you either do the sandwiches in a skillet with a lid, so either do them one at a time in a smaller skillet, or you can do them all at the same time if you have a skillet that fits them all.)

Cover the pan with a lid, and cook until the first side is toasted. Gently flip and cook, with the pan covered, until the second is toasted and the cheese is melty.

Let the sandwich sit for 2-3 minutes before slicing and serving.