Preheat grill. Brush both sides of each tuna steak with 2 teaspoons olive oil and season with salt and pepper. Place on the hot grill and cook for 2 minutes per side for rare. Saute onions and garlic in 2 tablespoon olive oil until soft and fragrant. Add tomatoes, olives, parsley and capers and simmer until tomato juice evaporates. Season to taste. Spoon sauce over tuna steaks.
You will probably not need to add additional salt to this dish as the olives and capers are quite salty enough.