In a bowl, combine cilantro, ginger, garlic, lime juice, soy sauce, sugar and 2 tablespoons of the olive oil. Stir together until well mixed. Coat a skillet with remaining olive oil and heat on medium high heat until pan is very hot (about two minutes). Season tuna steaks with salt and pepper. When pan is hot, add steaks and sear until done to your liking — one to three minutes per side (see doneness timetable below). Pour half the cilantro mix into pan and flip steaks briefly to coat fish. Remove from heat and plate immediately. Arrange slices of avocado on each plate, then spoon remaining cilantro sauce over each steak. Garnish with cilantro and lime, and serve with rice and green salad. Serves two.