Warm sauerkraut in a small saucepan over low heat.

Spread butter on one side of each slice of rye bread.

Flip bread slices over.

Place two slices of Gruyere cheese on four of the bread slices.

Divide the warmed sauerkraut between the cheese-topped bread slices.

Top with the remaining bread slices, buttered side up.

Place sandwiches in a large skillet or griddle pan over medium heat.

Cook until lightly browned on one side.

Flip and brown on the other side until cheese is melted.

Repeat the process with remaining sandwiches.

Slice each sandwich in half.

Serve with German mustard on the side (optional).