1/4 cup extra-virgin olive oil
1 tablespoon unsalted butter
1 yellow onion, coarsely chopped
3 garlic cloves, minced
1 to 2 jalapeños
1/4 teaspoon crushed red pepper flakes
4 cups chicken or vegetable stock
3/4 cup fresh basil leaves, packed
salt
pepper
Preheat the oven to 400°. Toss together the green tomatoes, 2 tablespoons of the olive oil, salt and pepper. Spread the tomatoes in an even layer on a baking sheet and roast for 30 minutes. In an eight-quart stockpot over medium heat, add the onions, jalapeños and garlic with the butter and remaining 2 tablespoons of olive oil. Cook until the onions have slightly caramelized, about 8 minutes. Sprinkle in the crushed red pepper flakes and stir to combine about 1 minute. Add the oven-roasted tomatoes and stock to the pot and bring to a boil. When the soup comes to a full boil, toss in the fresh basil leaves and wilt over medium heat. Simmer for 8 minutes. Working in batches, puree the soup in a blender until smooth, ensuring that the basil leaves are entirely pureed into soup. An immersion blender can also be used.