Preheat your oven to 350°F and lightly grease your baking dish. This ensures the casserole doesn’t stick and cooks evenly. Proper preparation saves time later.
In a large bowl, combine cooked rice, chopped corned beef, and cabbage. Mix everything well so each bite has a balanced flavor. This forms the hearty base of the casserole.
Melt butter in a saucepan and sauté shallots with garlic until soft. This step builds a deep, savory flavor. Don’t rush it—soft onions make a big difference.
Add flour and cook briefly, then slowly whisk in milk. Let each addition blend fully to avoid lumps. Simmer until thick and smooth.
Reduce heat and stir in most of the Swiss cheese, mustard, salt, and pepper. Mix until melted and creamy. This creates the rich signature sauce.
Remove from heat and stir in sour cream. Pour the sauce over the beef mixture and mix well. Transfer everything into the prepared baking dish.
Top with remaining cheese and cover with foil. Bake for 25 minutes, then uncover and bake until golden. The top should be bubbly and slightly browned.