Preheat oven to 425°F.
Cut the end off the heads of garlic, drizzle with EVOO and season with salt and pepper. Wrap them in foil and roast to soft, 40-45 minutes.
Reduce heat to 300°F. Arrange halved tomatoes on a cooking rack set on a baking sheet so heat can circulate around the tomatoes. Drizzle with EVOO and season with salt, pepper and herbs of choice. Roast 2 hours.
When tomatoes are done, sauté onions in EVOO, a turn of the pan, until soft, 10 minutes. Place roasted garlic cloves, half the roasted tomatoes, onions and canned tomatoes in a food processor, and purée until smooth. Place tomato base in a pot with the stock and stir to combine and heat through.