Heat the olive oil in a large saucepan or Dutch oven, over medium heat. Add the onion and red pepper flakes and cook, stirring occasionally, until the onion is tender, about 7-10 minutes. Add the tomatoes and 1 1/2 cups of water and cook, stirring occasionally, for about 30 minutes. Add the basil, season with salt and pepper, and let the soup cool briefly before pureeing the soup in a food processor, or a blender. Blend the soup in batches, and only fill the blender or food processor halfway for each batch. Be careful processing hot soup as the steam will need to escape. Pass the soup through a fine-mesh strainer, pressing on the solids with a ladle. Add the strained soup back to the saucepan and warm over a medium-low heat. Season with salt and pepper, to taste. Garnish with popcorn and sliced scallions.