1. Dice the onion and sauté it in a frying pan with half of the olive oil. Add grated tomato and cook for 30 minutes or more. Remove from pan and set aside.
2. In the same pan, add the rest of the olive oil and sauté the halved garlic cloves. Once done, remove garlic from oil, draining well.
3. Smash the garlic with a fork and pass it through a sieve. Lightly salt and set aside.
4. Cut tuna into large pieces and coat with flour. Sear the tuna in the frying pan with the oil left over from sautéing the garlic. Add the prepared tomato sauce and cook everything together for a few minutes.
5. Purée the tuna and tomato sauce with an immersion blender. Pass through sieve.
6. Serve by spooning the tuna mixture onto the plate in small portions and add a dollop of garlic purée on top of each tuna mound.
Tuna filet is not the only cut suitable for this recipe. It can also be made with different cleaned cuts of tuna.