8 slices rye bread, divided
1 bottle Kelchner’s Hot Mustard
1 lb. thinly sliced corned beef
8 oz. thinly sliced Fontina cheese
1/2 sweet onion (such as Maui or Vidalia), thinly sliced
4 T. (1/2 stick) butter, divided
Place 4 bread slices on work surface. Spread mustard on 1 side of each. Divide corned beef among bread slices. Top with cheese and onion. Cover with remaining 4 bread slices, pressing slightly to adhere.
Melt 2 tablespoons butter in each of 2 large nonstick skillets over medium heat. Place 2 sandwiches in each skillet and cook until golden brown on bottom, pressing occasionally with spatula, about 3 minutes. Turn sandwiches over and cook until golden on bottom and cheese melts, about 3 minutes. Transfer sandwiches to plates, cut in half, and serve.
Optional: Serve with horseradish coleslaw and dill pickle