Remove and wash outer leaves from cabbage; line a 13x9-inch baking dish with the leaves to cover the bottom entirely. Shred remaining cabbage.
Mix shredded cabbage, potatoes, and smoked sausage in a bowl; spread atop the cabbage leaves in the baking dish.
Whisk milk, eggs, salt, and pepper together in a bowl; add bread cubes. Set aside until bread absorbs some liquid, 1 to 2 minutes; pour over the cabbage mixture. Top with butter slices.
Bake in preheated oven until the potatoes are tender and the top is lightly browned, about 45 minutes.