Preheat the oven to 180°C.
Cut the onion into wedges, the tomatoes in half, then arrange them in a baking tray. Tuck the garlic cloves between them.
Season with salt and pepper, then drizzle with oil and vinegar.
Bake in the oven for 25-30 minutes, until the vegetables are tender.
Take the tray out of the oven and let the vegetables cool, then remove their skins.
Put the flesh of the veggies in a liquidiser together with cooking juices. Whiz until you get a rough sauce. Add the sundried tomatoes and herbs. Season with salt and freshly ground pepper.
Whizz again, until you get a thick, smooth or lightly textured sauce.
You can use this sauce immediately or chill/ freeze it for later use.