1. Chop onion, garlic and pepper and cook them slowly till golden in a casserole with olive oil. Add the tomato and the sugar.
3. Cover the tuna pieces with soy sauce and let them marinate for around 2 hours. Take them out from the soy sauce and let them rest in the fridge. Roast the tuna pieces in a pan on all the sides but letting them raw inside. Add sesame seeds during the process.
4. Take them out from the pan and cut in cubes. Use a good knife to ensure that the cut is clean. Serve the cubes with drops of marmitako sauce on the base and the tuna on the top and enjoy!