01 - Preheat your oven to 375°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - In a large skillet over medium heat, melt the butter. Add the onion and cook until soft, about 3 minutes. Add the garlic and cook 1 minute more until fragrant.
03 - Stir in the flour and cook for 1 minute to remove raw taste. Gradually whisk in milk and cream to avoid lumps. Continue whisking until the sauce thickens, about 3–4 minutes.
04 - Stir in salt, pepper, and paprika. Remove from heat. Add 1 cup mozzarella and 1/2 cup parmesan, stirring until melted and smooth.
05 - In a large bowl, combine the shredded cabbage, corned beef, and cheese sauce. Mix well until all ingredients are evenly coated.
06 - Spread the mixture evenly in the prepared baking dish. Top with the remaining 1/2 cup mozzarella and a generous sprinkle of additional parmesan cheese.
07 - Cover with foil and bake for 25 minutes until the cabbage begins to soften.
08 - Remove the foil and bake for another 15 minutes, until bubbling and golden on top with cheese melted and slightly browned.
09 - Let rest for 5–10 minutes before serving to allow the casserole to set. Garnish with chopped parsley if desired.