2Heat 6 tablespoons butter in a large skillet over medium heat; cook and stir onion in the melted butter until lightly browned, 10 to 15 minutes.
3Remove and set aside several large leaves of cabbage. Core cabbage and cut into very fine shreds. Mix cooked onion, chopped cabbage, eggs, and milk together in bowl; transfer mixture to a 9x13-inch casserole dish. Cover mixture with reserved whole cabbage leaves.
4Bake in the preheated oven until cabbage mixture is cooked through and set, about 50 minutes.
5Heat 2 tablespoons butter in a skillet over medium heat; cook and stir cracker crumbs in the melted butter until lightly browned, about 5 minutes.
6Remove cabbage leaves from top of casserole dish and sprinkle cabbage-egg mixture with buttered cracker crumbs.
7Bake in the preheated oven until topping is browned, about 10 more minutes. Season with salt and black pepper.