This is a traditional recipe from the little island of Carloforte (South West of Sardinia). It's flavoured and harmonic. Tuna fish is catched on Carloforte coasts. You can use any kind of short pasta, better if you have an artesanal one, but even the long varieties such as spaghetti, trenette, bavette are fine.
Let the finely chopped onion fry in a pan on low heating. It should not change colour, only melt.
When it's ready, turn the heat on and add the tuna fish cut into small cubes. If you use the fresh one let it pink inside and do not overcook it.
Turn the heat low again and add the peeled tomatoes (you should boil them for few seconds and then keep skins and seeds away). Taste and season with salt and let the tomatoes cook for 10 minutes.
Set out from the fire, let it rest for a while and then add the pesto.
Cook pasta al dente and add the sauce and some freshly ground black pepper. Serve immediately.
It's a regional and traditional recipe for pasta: delicious! Hope you enjoy it.