Roast: Preheat your oven to 200°C (400°F). Place the tomatoes, onions, and garlic in a big roasting pan. Roast until the vegetables are soft, sweet, and concentrated.
Deglaze: Don't lose the best part! When you take the tomatoes out of the oven, check the pan for brown, caramelized bits stuck to the bottom. This is pure flavor. Pour a little water or tomato juice onto the hot baking sheet to 'deglaze' it, scrape up all those sticky bits, and combine that liquid with everything else.
Char: While the tomatoes are roasting, char the bell peppers on a grill until they are blackened on all sides.
Steam: Place the hot, charred peppers into a big bowl and cover it tightly. Let them steam and soften completely.
Peel: Once they are cool to the touch, remove the charred peels, seeds, and cores.