Preheat the oven to 375°F.

1. Lightly butter a 2-quart baking dish.

Sauté the cabbage:

1. Place a deep 12-inch sauté pan or Dutch oven over medium heat. Melt 2 tablespoons of the butter, then add the onions. Cook, stirring often, until the onions are softened, about 5 minutes.
2. Add the shredded cabbage and stir. Cook, stirring often, until the cabbage is wilted and lightly browned in spots, about 10 minutes.

Meanwhile, make the topping:

1. While the cabbage cooks, coarsely crush the crackers, either by pulsing them in the food processor or by placing them in a zip-top bag and crushing with a rolling pin. You should have about 1 1/2 cups crumbs.
2. Transfer the cracker crumbs to a medium bowl. Melt the remaining 2 tablespoons butter and drizzle over the crumbs, toss to coat. Add 1 cup of the cheddar and toss to mix; set aside.

Assemble the casserole:

1. When the cabbage is soft, stir the cream of celery soup and milk together in a measuring cup, then stir the mixture into the cabbage. Stir in the remaining cheddar cheese, salt, and pepper and remove the pan from the heat.
2. Spread the mixture into the prepared baking dish and top evenly with the crumbs.

Bake:

1. Bake until bubbling and lightly browned on top, about 35 minutes. Let cool for 5 minutes before serving.
2. Leftovers keep in an airtight container in the refrigerator for up to 4 days.