Heat the olive oil in a large pot over medium heat.
Add the chopped onion and celery. Cook for about 8 minutes until softened.
Stir in the tin of tomatoes and the brown sugar. Cook for 2–3 minutes to combine flavours.
Add the green beans, kidney beans, and butter beans.
Pour in the vegetable stock, ensuring the beans are covered.
Simmer for 20 minutes, stirring occasionally.
Add the chopped fresh basil and adjust the seasoning with salt and pepper as needed.
Serve hot with a pile of fresh watercress on top for added freshness.