Cook the pasta according to packet instructions (approx. 10 minutes), then drain and keep warm.
Meanwhile, make the sauce. Finely chop the onion, garlic, celery, carrot and red pepper.
Heat a splash of olive oil in a saucepan over low heat. Add the onion, carrot and celery, and gently fry for about 5 minutes until softened.
Stir in the red pepper, capers and oregano. Cover with a lid and sauté for 8minutes, stirring occasionally.
Add the tomato purée and stir to coat the vegetables well. Pour in the passata and enough water to cover everything well. Season with salt and pepper.
Bring to the boil, then reduce to a gentle simmer. Cover and cook for 20 minutes until the vegetables are tender.
Stir in the parsley, then turn off the heat. Use a stick blender to purée the mixture until smooth and creamy. Add more water, if it’s too thick.
Fold in the kale or spinach and bring back to the boil. Cook for 3 minutes or so.
Gently fold in the sardines, then serve with the pasta.