ROAST TOMATOES: Preheat the oven to 400°F. Add 3 pounds halved Roma tomatoes to a large bowl. Toss with ¼ cup extra virgin olive oil, 1 tablespoon kosher salt, and 1 teaspoon fresh ground black pepper. Spread in a single layer on a rimmed baking sheet. Roast for 45 minutes, until soft, blistered, and lightly caramelized.
COOK AROMATICS: Place an 8-quart stockpot over medium heat. Add 2 tablespoons unsalted butter and melt. Add 1 diced sweet onion, 1 tablespoon minced garlic, and ¼ teaspoon crushed red pepper flakes. Cook for about 10 minutes, stirring occasionally, until the onions are soft and lightly golden.
BUILD SOUP: Add the roasted tomatoes and all juices from the pan, 2 cans diced tomatoes, 2 tablespoons dried basil leaves, 1 tablespoon Italian seasoning, remaining 1 tablespoon kosher salt, and 32 ounces chicken stock. Stir well.
SIMMER: Bring the soup to a boil, then reduce heat to medium-low. Simmer uncovered for 40 minutes, stirring occasionally, until the flavors deepen.
BLEND: Carefully transfer the soup to a blender in batches, filling no more than halfway. Blend until smooth. Pour the blended soup through a fine-mesh sieve back into the pot to remove skins and seeds.
SERVE: Warm gently over low heat if needed, then serve hot.
Notes
Make-Ahead: This soup can be made up to 3 days ahead. The flavor improves as it rests.Storage: Store in an airtight container in the refrigerator for up to 4 days.Reheating: Reheat gently on the stovetop over medium-low heat, stirring often.Freezing: Freeze for up to 3 months in freezer-safe containers. Thaw overnight in the refrigerator before reheating.Ingredient Note: Roma tomatoes give the best concentrated flavor, but vine-ripened tomatoes can be used if needed.