Preheat the oven to 180°C/350°F/ Gas 4. Heat the oil in a heavy saucepan over a low heat and cook the onion and garlic for about 20 minutes until soft and golden.
Spread over the base of a 30 cm/12 in shallow baking dish.
Cut the tomatoes crossways into 6 mm/ 1/4 inch thick slices. (If the tomatoes are very large, cut the slices in half.)
Cut the zucchinis diagonally into slices about 1 cm / 1/2 inch thick.
Arrange alternating rows of zucchinis and tomatoes over the onion mixture and sprinkle with herbs, cheese and salt and pepper.
Drizzle with olive oil, then bake for 25 minutes until the vegetables are tender.