1 large leek, white part only, cut into 1cm/陆in slices
2 banana shallots, peeled, sliced into rings
3 tbsp hot chicken stock
1 tsp plain flour
陆 tsp smoked paprika (available from Spanish delicatessens)
1陆 tsp salt
250ml/9fl oz vegetable oil, for deep frying, plus 1 tbsp extra
25g/1oz unsalted butter
110g/4oz cooking chorizo, skin removed, chopped
2 mackerel fillets, skin scored several times using a sharp knife