In a large ceramic bowl combine the mangoes, onions, jalapeno, ginger, Sriracha, and lemon juice, and blend well. Season with salt and pepper. Use or refrigerate.
Meanwhile, heat to high heat a non-stick skillet. Once the skillet is hot, rub some olive oil over the yellowfin tuna and place the oiled side into the pan. Sear on high for 1 ½ to 2 minutes per side.
Remove from the skillet when done: they should be pink in the middle, sprinkle more olive oil over it, and gently squeeze lemon over each tuna.
Spread some of the spicy mango salsa in the middle of the plate and place the yellowfin tuna over it.