Preheat oven to 400° F.
Toss tomatoes in 2 tablespoons olive oil, salt, and pepper. Spread on baking sheets and roast for 40 minutes.
In a large stockpot, sauté the onions and garlic with 2 tablespoons olive oil, butter, and red pepper flakes until the onions start to brown. Add the tomatoes and remaining ingredients.
Bring to a boil then simmer uncovered for 40 minutes. Puree with an immersion blender or in batches in an upright blender.
Season to taste and serve topped with crème fraîche, basil leaves, chickpeas, and/or croutons.