01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - Melt butter in a large skillet over medium heat. Add diced onion and cook until softened, approximately 3 minutes. Add minced garlic and cook for 1 additional minute until fragrant.
03 - Sprinkle flour into the skillet, stirring constantly for 1 minute to cook out raw flour taste.
04 - Gradually whisk in whole milk and heavy cream, ensuring no lumps form. Continue whisking until sauce thickens, about 3-4 minutes. Stir in salt, black pepper, and paprika. Remove from heat and add 1 cup mozzarella with 1/2 cup parmesan, stirring until completely melted and smooth.
05 - In a large mixing bowl, combine shredded cabbage, chopped corned beef, and prepared cheese sauce. Mix thoroughly until cabbage is evenly coated.
06 - Spread mixture evenly into prepared baking dish. Top with remaining 1/2 cup mozzarella and generous sprinkle of additional parmesan cheese.
07 - Cover baking dish with aluminum foil and bake for 25 minutes.
08 - Remove foil and continue baking for 15 minutes until cheese is bubbly and golden brown.
09 - Let casserole rest for 5-10 minutes before serving to set. Garnish with chopped fresh parsley if desired.