Mackerel à la Lyonnaise

From ‘The Pauper’s Cookbook’ by Jocasta Innes, a mainstay when I was a student. Serves two.

2 large or 4 small mackerel, filleted
salt and pepper
2 large onions
breadcrumbs
2 tbs white wine vinegar
3 tbs cider, white wine, or water
2 oz butter

Peel and thinly slice the onions. Melt most of the butter in a saucepan over low heat, add the onions, cover, and stew gently until soft and pale gold, stirring from time to time. When they are ready, add the vinegar. Spread half the onions on the bottom of an ovenproof dish and arrange the mackerel on top. Season with salt and pepper, add the rest of the onions, and pour over the cider, wine, or water. Sprinkle with breadcrumbs, dot with butter, and bake in a fairly hot oven for about 20 minutes, until the fish is done. Serve with baked or steamed potatoes and perhaps a green vegetable.