Preheat oven to 220C/375F/ Gas No 5. Put the cherry tomatoes, garlic, 2 onions & basil onto a tray & drizzle with 2 tbsp oil. Sprinkle with sea salt & black pepper. Bake for 30- 40 minutes until the tomatoes are browned.
Mix the pesto, 3 tbsp olive oil & bread together in a bowl, ensuring the bread is evenly covered. Spread the croutons onto a baking tray & bake for 5 minutes in the oven with the tomatoes. Turn the croutons & bake for a further 5 minutes. Put to one side.
Put the rest of the olive oil in a pan & gently fry the onions for 10- 15 minutes, add the vinegar & sugar. Cook for a further minute.
Liquidize the roast tomato mixture. Add the fried onion & stock. Reheat the soup & check the seasoning.