Heat your oven to 200°C/Fan 180°C/Gas 6. Slice the tomatoes into 1cm thick rounds.
Halve, peel and thinly slice the onion. Strip the leaves from the thyme.
Gently mix all the prepared veg and herbs together in a large bowl with a pinch of salt and pepper and 1 tbsp balsamic vinegar.
Rub 1 tsp olive oil into a small ovenproof frying pan or baking dish. Arrange the tomatoes and onions in the dish in overlapping layers. Your tian should be about 3 tomato slices deep. Drizzle 1 tsp oil over the top, scraping in any excess juices from the bowl.
Roast the tomato tian on the top shelf of your oven for 35 mins or till the tomatoes are caramelised and golden around the edges and on the top.
Zest the lemon and squeeze out 1 tbsp juice. Whisk or shake in a jam jar with 1 tsp olive oil and a pinch of salt and pepper.
Halve and de-stone the avocado. Strip the peel off. Chop the flesh into 1-2cm thick slices. Pluck out any woody stalks from the watercress. Toss the avocado and watercress with the dressing. Serve alongside your roast balsamic tomato tian.