This is adapted from Cook's Illustrated. The original recipe calls for 3/4 pound beef and 3/4 pound bratwurst, but I adjust that to 1 pound beef and 1/2 pound bratwurst, since it's easier to buy beef in 1-pound quantities. It's very good!
Heat oven to 350. Place cabbage in bowl, wrapped with plastic wrap, and microwave until outer leaves are pliable and translucent, 3 to 6 minutes. Carefully remove outer leaves and set aside. Replace plastic and repeat until you have 15 to 17 large, intact leaves.
Heat oil in Dutch oven over medium-high heat until shimmering. Cook onion about 5 minutes. And garlic, ginger, cinnamon, and nutmeg and cook about 30 seconds. Transfer half of mixture to small bowl and reserve. Off heat, stir tomato sauce, sugar, vinegar, ½ teaspoon salt, and ¼ teaspoon pepper into remaining mixture in Dutch oven.
Pulse bread and milk in food processor to form smooth paste. Add reserved onion mixture, beef, bratwurst, ½ teaspoon salt, and ¼ teaspoon pepper, and pulse to combine.
Using a sharp knife, trim tough ribs from cabbage leaves by making one slice down the length of the left and right side of each rib. Roll 2 tablespoons of meat mixture into each leaf and arrange seam-side-down in 9x13-inch baking dish. Pour sauce over rolls, cover with foil, and bake 45 minutes. Remove foil and bake 15 more minutes to slightly thicken sauce. Serve.