Peel and finely chop the onion. Place a large saucepan over a medium high heat. Add 2 tablespoons of oil, the onion, anchovies and chopped bell pepper and fry for 5 to 10 minutes, or until softened, stirring occasionally. Meanwhile, place a large pan of water on to boil, add a pinch of salt and cook the pasta according to the packet instructions.
Chop the zucchini and tomatoes on small cubes. Add the chopped tomatoes to the veggies, also add the capers and finely chopped garlic, fry for about 2 minutes. Add small zucchini cubes and 1 handful of basil, fry for another 2 minutes. When the pasta is ready, drain and reserve 1 cup of water. Add drained tuna to the veggies, pour over tomato passata, stir well. Season with salt and a tiny pinch of black pepper.
Lower the heat, add the drained pasta and reserved water if necessary. Stir well and remove from the heat. Add sour cream or creme fraiche and stir once again. Check the seasoning and season with salt and pepper if necessary. Add a handful of fresh basil and serve.