Heat oven to 375°F (190°C).
Spread rye bread slices on a sheet pan and brush both sides with melted butter. Toast in the oven for 6–8 minutes until lightly golden. This prevents a soggy casserole.
Drain sauerkraut very well. Squeeze by hand or in a clean towel to remove excess liquid.
Whisk mayonnaise, ketchup, relish, horseradish, Worcestershire, paprika, onion powder, salt, and pepper. Taste and adjust horseradish and salt.
Lightly grease the baking dish.
Arrange a single layer of toasted rye, cutting pieces to fit. Spread a thin layer of dressing over the bread.
Add half the corned beef, then half the sauerkraut, and a sprinkle of caraway seeds if using.
Top with half the Swiss cheese.
Repeat layers: bread + dressing, remaining beef, remaining sauerkraut, caraway, then the rest of the Swiss.
Cover loosely with foil and bake for 20 minutes.
Remove foil and bake another 10–15 minutes until the cheese bubbles and browns at the edges.
Let the casserole rest for 10 minutes before slicing. This helps it set.
Drizzle extra dressing over the top or serve on the side.
Garnish with chopped dill pickles or fresh dill for brightness.
