stir together cornstarch and 1 1/2 tbsp water until smooth, set aside.
add pineapple and 2 tbsp water to blender and puree.
in a saucepan add pineapple, soy sauce, sugar, ginger, bring to a boil.
add cornstarch, stir 1 minute.
add salt& pepper to tuna, cook on oiled grill, turning once about 5 mins per side, or to taste spread pineapple sauce on each tuna steak and grill,covered, 1-2 mins more, serve mix wasabi powder, water and mayo together, serve alongside tuna.