Preheat oven to 350 degrees. Spray a 9x13 baking dish with non-stick cooking spray.
In a large bowl, mix together rice, cabbage and corned beef.
In a large saucepan, melt the coconut oil or shortening over medium heat. Add the garlic and chopped shallots. Saute for 4-5 minutes or until softened.
Whisk the flour into the onion mixture and cook for 1 minute. Slowly whisk in the milk letting each prior amount added fully incorporate into flour mixture before adding more. Bring to a boil and simmer, whisking constantly, for about 5 minutes or until thickened.
Turn heat to low. Stir in 1½ cups shredded Swiss cheese, mustard, salt and pepper. Whisk to combine.
Remove pan from heat and stir in sour cream. Pour sauce over cabbage mixture and stir well. Pour mixture into prepared dish and top with remaining shredded Swiss cheese.
Cover dish with foil. Bake at 350 degrees for 25 minutes. Remove foil and bake for an additional 5-10 minutes until cheese is browned.
1) This casserole freezes really well. Simply split the ingredients between two dishes or double the ingredients. Then, wrap tightly in foil and freeze for up to 2 months. Thaw in refrigerator overnight before baking for best results.