Heat medium skillet over medium heat; add 2 tablespoons butter, onions and salt. Cook onions until softened, golden brown and caramelized, about 10 minutes. Add water to pan if needed while cooking. Remove onions and reserve. Place kielbasa slices in pan and fry over medium-high until browned around the edges, about 5 minutes. Set aside.
In small bowl, combine horseradish and sour cream. Butter 1 side of each bread slice. Place two slices, buttered-side-down, in large skillet over medium heat. Add 2 slices of cheese to each, followed by mashed potatoes. Top with sauerkraut, onions, kielbasa and two additional slices of cheese. Spread horseradish sauce on unbuttered side of 2 remaining bread slices. Place bread buttered-side-up on top of each sandwich. When bottom slice is golden and crisp, flip sandwiches to grill remaining side. Cover with lid and continue to grill until bread is toasted, ingredients are warmed through and cheese is melted.
- 6 tablespoons butter, divided
- 3/4 cup sweet onions, sliced
- 1/2 teaspoon salt
- Water as needed
- 10 ounces kielbasa sausage, cut in 1/4-inch slices
- 1-2 tablespoons prepared horseradish
- 5 tablespoons sour cream
- 4 slices marble rye bread
- 8 slices Wisconsin Aged Cheddar cheese
- 1/2 cup mashed potatoes, at room temperature
- 1/2 cup sauerkraut, liquid drained, at room temperature