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Mix all the crumb ingredients together in a bowl with a pinch of salt and set aside.
Heat oven to 180C/160C fan/gas 4. Heat the duck fat in a large flameproof roasting tin. Sear the cabbage wedges in the tin until the cut sides are crisp and slightly burnt, then turn them so they are cut-side up and scatter over the crumb mix. Dot over the butter and bake for 15 mins until the crumb is golden.