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