Heat the olive oil in a large soup pot over medium high heat. Add the onion and cook, stirring often, until softened, about 10 minutes. Add the tomatoes and the stock. Bring to a boil and then reduce to a simmer. Cook until soup is slightly thickened, about 20 minutes. Season with salt and pepper. Stir in the basil. Place the pot of soup directly in sink, and using a stick blender, blend until smooth. Serve immediately.