Preheat oven to 350 degrees. Spray a 9x13 baking dish with cooking spray.
Combine rice, cabbage and corned beef in a large bowl.
In a large pan, melt butter over medum heat. Add shallot and garlic and saute for 3-4 minutes.
Whisk in flour and cook for 1 minutes. Slowly add in milk letting each prior amount added fully incorporate into flour mixture before adding more. Bring to a bowl and simmer, whisking constantly, for about 5 minutes or until thickened.
Turn heat to low. Stir in 1 ½ cups shredded Swiss cheese, mustard, salt and papper.
Remove from heat and stir in sour cream. Pour sauce over cabbage mixture and stir well. Pour 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.