Finely slice 250 g (9 oz) onion and blanch for a few minutes in boiling water. Drain well and cook gently to a golden colour in a little oil. Moisten with 1½ dl (5 fl oz or ⅝ U.S. cup) white wine and reduce by two-thirds then add 500 g (1 lb 2 oz) roughly chopped flesh only of tomato which has been sauteed in a little oil. Season well and cook together gently.
Place the finished sauce in a gratin dish and arrange 12 nice cleaned sardines on top; sprinkle with white breadcrumbs mixed with a few crushed fennel seeds and some olive oil and gratinate in a hot oven.