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 soft, about 3 minutes. Add minced garlic and cook 1 minute more until fragrant.
03 - Stir flour into the skillet and cook for 1 minute to remove raw taste. Gradually whisk in whole milk and heavy cream, continuing to whisk until smooth. Simmer for 3-4 minutes until sauce thickens noticeably.
04 - Stir in salt, black pepper, and paprika. Remove from heat. Add 1 cup mozzarella cheese and 1/2 cup parmesan cheese, stirring until completely melted and sauce is velvety smooth.
05 - In a large mixing bowl, combine shredded cabbage, chopped corned beef, and the prepared cheese sauce. Mix thoroughly until all ingredients are evenly coated.
06 - Spread mixture evenly in the prepared baking dish. Top with remaining 1/2 cup mozzarella cheese and a generous sprinkle of additional parmesan cheese for a golden crust.
07 - Cover baking dish tightly with aluminum foil. Bake for 25 minutes to allow flavors to meld and cabbage to soften.
08 - Remove foil and bake for an additional 15 minutes until cheese is bubbling and golden brown on top.
09 - Let casserole rest for 5-10 minutes before serving to allow flavors to settle. Garnish with chopped fresh parsley if desired.