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