Place a roasting pan on top of the stove over a medium high heat. Add the red onions, garlic and cayenne pepper, drizzle with enough olive oil to coat, and cook for 3-4 minutes.
Add the tomatoes cut side down, season with a pinch of salt, pepper and the sugar, drizzle over the balsamic vinegar and cook over a high heat for 3-4 minutes, until the tomatoes begin to caramelize. Stir well, and then transfer to the oven for 20-25 minutes.
Return the roasting pan to the top of the stove. Over a medium high heat add the vegetable or chicken stock, bring to the boil and then simmer for 4-5 minutes. Add the double cream, stir well and continue to simmer for 2-3 minutes.
Using a stick blender or potato masher, blend the soup until almost smooth but still with a couple of chunky bits (use a potato masher for a more rustic texture). Serve warm with dollops of pesto on top.
Sun-dried Tomato Pesto
Toast the pine nuts in a dry frying pan until golden.
Put the sundried tomatoes into a mortar and pound with a pestle until completely broken down. Season with a touch of salt, add the pine nuts and continue to pound until you reach a pesto consistency.
Stir in the Parmesan, and then add enough olive oil and sundried tomato oil to bring the pesto to a spooning consistency (it should provide 3–4 tbsp).