Preheat a nonstick griddle or a grill pan to high heat.
Cube tuna into bite size pieces and place in a food processor. Pulse the processor to coarse-grind the tuna; it should take on the consistency of ground beef or turkey. Transfer tuna to a bowl. Combine ground tuna with garlic, ginger, dark soy sauce, scallions, red bell pepper, sesame oil, seasoning blend and black pepper. Form 4 large patties, 1 1/2 inches thick. Drizzle patties on both sides with light-in-color oil, such as canola, safflower or peanut oil.
Cook tuna burgers 3 minutes on each side for rare, up to 6 minutes each side for well done. Preheat broiler.
In a medium nonstick skillet over high heat, sauté fresh shiitakes for 2 or 3 minutes in 1 tablespoon light oil—1 turn of the pan in a slow stream. Season mushrooms with salt and pepper.
Open up the rolls and lightly toast them under the broiler. Spread mango chutney on the bottom of each bun. Top with tuna burger, shiitakes, red leaf lettuce. Spread wasabi mustard or sweet hot mustard on bun tops and set them in place. Serve with exotic chips and pickled ginger, if desired.