1 large sweet yellow or red onion, peeled and cut into chunks
2 bell peppers (anything but green), seeded and cut into chunks
3-4 cloves garlic, peeled and minced or pressed
3 tablespoons good olive oil
1-1/2 teaspoons kosher or sea salt
1/2 teaspoon freshly ground black pepper
1-1/2 tablespoons tomato paste
Preheat the oven to 400 degrees.
Toss everything except the tomato paste together in a large bowl until the vegetables are fully coated with the olive oil and seasonings. Spread them out in a large baking dish or on a rimmed baking sheet.
Roast for 45 minutes, tossing once halfway through. Vegetables should be lightly browned, tender, and juicy.
Remove from oven and cool slightly. Place in a food processor, add the tomato paste, and puree briefly, just until blended. Stop to scrape down the sides of the bowl during processing if needed.