Sauté onions in olive oil.
Chop up equal amounts of aubergine/eggplant, courgettes/zucchini, and coloured peppers/capsicums.
Spread the sautéed onions over the bottom of a roasting pan or gratin dish.
Layer in the vegetables, seasoning with salt and pepper as you go.
Lay sliced tomato on top.
Drizzle over some olive oil
Bake in a very hot oven (about 400°F/200ºC or 450°F/220°C, depending on how your oven behaves) until the liquid has evaporated and the vegetables have sunk into a deep-coloured confit, about 2 hours stirring once or twice.
(Check the tian at about half-time. If you find the top browning too quickly, lay on a piece of foil and continue cooking.)