Combine the first four ingredients in a small bowl; store in the fridge until needed. Preheat your grill to medium-high. Brush the tuna with the oil; season with pepper. (Lightly oil the grill if not non-stick.) Grill the tuna 2 to 3 minutes per side, or until seared on the outside and rare in the middle (if you prefer your tuna entirely cooked, add 1 to 2 minutes cooking time per side). Set the tuna on a plate, cool to room temperature, and then cut into 1/2 inch strips. Spread one side of a tortilla with 1/3 of the wasabi mayo. Place one green onion and 1/3 of the tuna and red pepper strips in rows down the centre of the tortilla. Roll into a tight cylinder, wrap and chill in the fridge for 30 minutes. Repeat with remaining tortillas and fillings. When chilled, slice the tortilla rolls into rounds, arrange on a tray and serve.