Heat a pan on medium-low heat. Butter 2 slices of bread. Spread the mustard relish on the other side of the slices of bread and place them butter side down in the pan.

Divide half the cheese, apples, sauerkraut, and remaining half of the cheese on each slice of bread.

Butter the remaining 2 slices of bread and place them butter side up on the sandwiches.

Brown the sandwiches about 5 minutes until the cheese starts melting and use a spatula to flip the sandwiches and brown them an additional 5 minutes.