In a medium bowl stir to combine mango, pineapple, avocado, lime juice, habanero, and ¼ teaspoon salt. Set aside.
Prepare a hot charcoal grill fire or preheat a gas grill on high heat for 10 minutes.
Sprinkle tuna steaks with remaining ½ teaspoon salt and pepper. Grill tuna steaks, turning once, until browned on both sides but still pink in the middle, 2-3 minutes per side.
If using wasabi paste, in a small bowl stir to combine mayonnaise and wasabi.
Spread both sides of each burger bun with mayonnaise. Top bottom of each bun with 2 lettuce leaves and a tomato slice. Place a tuna steak on each. Top with a big spoonful of pineapple-mango salsa, and finish with top half of buns.