Chop the cabbage into bite-sized pieces and finely dice the onion. Set aside. Preheat your oven to 350°F (175°C).

In a large skillet, melt the butter over medium heat. Add chopped onion and sauté for 3–4 minutes until slightly softened. Add the cabbage, season with salt and pepper, and cook for about 10–12 minutes, stirring occasionally. The cabbage should be tender with lightly browned edges.

Once the cabbage mixture is softened, remove from heat. Stir in the cream of chicken or mushroom soup until everything is well coated and creamy.

Transfer the mixture to a greased 9x13-inch baking dish. Use a spoon or spatula to spread it evenly.

Cover the dish with foil and bake at 350°F for 20 minutes. Then uncover and bake for another 10–15 minutes, until the top starts to brown slightly and the edges bubble. The casserole should have a golden, cheesy crust with soft, buttery cabbage underneath.

Let the casserole rest for 5 minutes before serving. This allows it to set slightly, making it easier to cut and serve.