Slice the onion into half-moons and saute in a pot with the olive oil over medium-low. Cook until tender. Add tomatoes, stock and basil. Turn heat up to medium and bring to a boil. Simmer for about 8-10 minutes. Season with salt and pepper.
At this point I use my immersion blender to pulse some of the chunks away, but not all. I like a my soups a little chunky so I can really enjoy the tomatoes.