Step 1

Arrange a rack in the middle of the oven and heat to 425°F.

Step 2

Line a rimmed baking sheet with parchment paper. Remove the whole tomatoes from the cans (reserving the juice), cut them in half if large, and lay them out in a single layer on the baking sheet. Scatter the garlic cloves and thyme sprigs over the tomatoes and drizzle with 1 tablespoon of the oil. Season with salt, pepper, and a pinch of red pepper flakes, if using. Roast until soft and caramelized, 30 to 35 minutes.

Step 3

Meanwhile, heat the remaining 1 tablespoon of oil in a Dutch oven over medium-high heat until shimmering. Add the onion, season with salt and pepper, and sauté until translucent and softened, about 5 minutes. Add the tomato paste and sauté until the paste has deepened in color, about 2 minutes more. Remove from the heat.

Step 4

When the tomatoes are ready, discard the thyme sprigs. Transfer the tomatoes and garlic cloves to the pot, then add the reserved tomato juice in the cans and the vegetable broth. Stir to combine and bring to a boil. Reduce the heat to maintain a gentle simmer, cover the pot, and simmer 20 minutes.

Step 5

Purée the soup in batches in a stand blender or with an immersion blender until smooth. Season with salt and pepper as needed.