ooo yum!
idea number one - fry loads of onions really gently until melting in lots of olive oil and some garlic if you like. Bung the tuna in on top and saute the whole lot with the lid on for about half an hour. Can add a bit of tomato puree to this sauce.
idea number two - marinate them in half balsamic vinegar/half soy sauce (about 3 or 4 table spoons of each) and a pinch of ginger for 15 mins. Then fry the steaks in a little olive oil and add the vinegar soy sauce mixture at the end of cooking.