1. Heat the oil in a large pan, add the onions, cover and gently cook for 15 minutes. Take off the lid, turn up the heat and cook, stirring, for 5 minutes until the onions begin to caramelise. Stir in the thyme leaves and set aside.
2. Preheat oven to 220°C. Put a heavy-based 24cm round casserole dish or ovenproof frying pan over low heat. Add the sugar and 50ml water. Heat gently, stirring, until the sugar dissolves, then turn up the heat. Allow the sugar to caramelise. (Swirl the pan occasionally.) When the sugar is deeply caramelised (honey-coloured), carefully add butter and vinegar. Stir to combine and take pan off heat.
3. Arrange tomatoes, cut-side down, in a single layer in the base of the pan or casserole. Cover with the onions. Unroll pastry and cut out a circle 1cm larger than the base of the casserole or pan. Position pastry on top of onions, and tuck in edges.
4. Cook for 30 to 40 minutes or until pastry is a deep golden colour. Leave to rest for 5 minutes. Invert onto a plate, garnish with thyme leaves and serve with a green salad.

Each serving 1 997kJ, 28g fat (19g saturated), 50g carbohydrate (18g total sugars).