Cut the tuna into 1-inch chunks and place in the bowl of a food processor. Pulse on and off 4 or 5 times until tuna is coarsely ground. Transfer to a mixing bowl and add bread crumbs, egg, onions, hoisin sauce, cilantro, gingerroot, garlic, sesame oil, salt and pepper. Mix gently using your hands. Form tuna mixture into 6 patties, about ½ inch thick. Wet your hands to prevent sticking, if necessary. Place patties on a parchment-lined pan and stick it in the freezer while you grill the pineapple. Lightly oil a grill pan and place over medium-high heat. Grill pineapple and onion rings until onions have softened and both sides have nice grill marks. (Alternatively, use your outdoor gas barbecue, which I prefer for any type of grilling.) Cover and keep warm. Re-oil the grill pan and cook burgers over medium-high heat for about 3 minutes per side, or until cooked to desired degree of doneness. Don’t overcook tuna burgers or they’ll be dry. Serve hot patties topped with grilled pineapple and onion rings and any other condiments that suit your fancy. Per burger: 231 calories, 5.5 g total fat (1 g saturated fat), 28 g protein, 16.8 g carbohydrate (2.4 g fibre, 9 g sugars), 84 mg cholesterol, 348 mg sodium