Scale and empty the bream, then sever the gills with a sharp knife and extract them without damaging the head. Put it on a board and make three neat incisions with a knife transversely in the thickest part of the fish on one side, as far as the backbone. Put salt, a little oil and a slither of garlic in each slit, then close with a slice cut from a lemon to fit. Sprinkle with salt, pepper and olive oil when you have set the fish on an oiled enamel plaque ready for the oven.
Slice the large onion into very thin rounds and lay them on the plaque round the fish. Slice the potatoes about ½″ thick and lay them on the onions. Sprinkle with thyme, savory and a little salt. Peel the tomatoes, squeeze out the pips, lay in pieces on the potatoes, sprinkle with oil. Put the plaque fairly high in the oven, preheated and hot, and bake for ¾ hour. The vegetables cook in the oil and juices released by the fish.