Cook the pasta according to the instructions on the packet.
Meanwhile, heat the oil in a saucepan, add the onion and fry gently until soft. Add the garlic and tomatoes and fry for a further 3–4 minutes until the tomatoes just start to break up. Season the sauce with salt and pepper and a little sugar if it is needed.
Stir the spinach into the sauce. Gently stir in the tuna, trying not to break it up too much, then drain and stir in the pasta. Drizzle a little more olive oil over the dish before serving.