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Heat oven to 200C/180C fan/gas 6. Snip the tops off the sausages and squeeze the meat out into a bowl. Stir in the fennel seeds, chilli flakes and half the zest, then shape into 4 patties. Place the burgers on a baking sheet and cook for 25-30 mins until golden and cooked through.
Meanwhile, mix the remaining zest and juice with the mayo and seasoning, then stir in the onion and fennel. Leave to stand while the burgers finish cooking.
Place a burger in each roll with some slaw, serving any extra on the side.