Adjust oven rack to the middle position and heat oven to 375°F. Place cabbage in a large bowl, cover tightly with plastic wrap, and microwave until the outer leaves become pliable and translucent, 3 to 6 minutes. Using tongs, carefully remove the softened outer leaves and set aside. Re-cover the bowl and repeat until you have 15 to 17 large, intact leaves.

Heat oil in a Dutch oven over medium-high heat until shimmering. Add onion and cook until golden, about 5 minutes. Stir in garlic, ginger, cinnamon, and nutmeg, cooking until fragrant, about 30 seconds. Transfer half of the onion mixture to a small bowl and set aside. Off heat, stir tomato sauce, sugar, vinegar, ½ teaspoon salt, and ¼ teaspoon pepper into the pot with the remaining onion mixture until the sugar dissolves.

In a food processor, pulse bread and milk into a smooth paste. Add the reserved onion mixture, beef, bratwurst, ½ teaspoon salt, and ¼ teaspoon pepper, then pulse until well combined, about ten 1-second pulses.

Trim the tough ribs from the cabbage leaves. Place 2 heaping tablespoons of the meat mixture in each leaf, roll tightly, and arrange seam-side down in a 13-by-9-inch baking dish. Pour the sauce over the cabbage rolls, cover with foil, and bake until the sauce is bubbling and the rolls are heated through, about 45 minutes. Remove the foil and bake uncovered until the sauce thickens slightly and the cabbage is tender, about 15 minutes. Serve.