Heat oven to 400°F.
Slice the cabbage 1/2 inch thick. Place 1/3 of the cabbage in a Dutch oven or casserole.
From each sausage, trim 1 end and, starting at the other end, squeeze to remove the meat from the casing.
Arrange half the sausage on top of the cabbage. Press firmly. Top with half the remaining cabbage and all the remaining sausage and press again. Spread the remaining cabbage over the sausage. Cover with foil and bake until the cabbage is tender, about 1 hour, 40 minutes. Remove from oven. Let stand for 5 minutes.
Meanwhile, in a small bowl, combine the sour cream and horseradish or mustard. Slice the casserole into wedges and serve with the sour cream mixture.