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, about 3 minutes. Add minced garlic and cook for 1 additional minute until fragrant.
03 - Stir flour into the butter and onion mixture, cooking for 1 minute to remove raw flour taste.
04 - Gradually whisk in whole milk and heavy cream, continuing to whisk until smooth. Cook for 3-4 minutes until sauce thickens enough to coat the back of a spoon. Stir in salt, black pepper, and paprika.
05 - Remove skillet from heat. Add 1 cup mozzarella and 1/2 cup parmesan cheese, stirring until completely melted and incorporated into smooth sauce.
06 - In a large mixing bowl, combine shredded cabbage, chopped corned beef, and cheese sauce. Mix thoroughly until cabbage is evenly coated.
07 - Transfer mixture to prepared baking dish, spreading evenly. Top with remaining 1/2 cup mozzarella and additional parmesan cheese as desired.
08 - Cover baking dish with aluminum foil. Bake for 25 minutes to begin cooking cabbage and melding flavors.
09 - Remove foil and continue baking for 15 minutes until cheese is melted, bubbly, and golden brown on top.
10 - Let casserole rest for 5-10 minutes before serving to allow flavors to settle. Garnish with chopped fresh parsley if desired.