Lightly brush non-stick baking sheet with butter. spread mustard on one side each breadslice. Arrange 6 slicesbread on baking sheet. Top each with cheese, 1 tablespoon sauerkraut, ham, then cover with remaining bread.
Brush remaining butter over sandwiches. Heat oven to 425F. Bake sandwiches about 10 minutes, flip, return to oven and bake 5 minutes more. Servewarm or at room temperature.