Preheat oven to 400 degrees F. Coat a large baking sheet with cooking spray.
Arrange wonton wrappers on prepared baking sheet and spray with cooking spray. Season the tops with salt, black pepper and mustard powder, to taste. Bake 6 minutes, until golden brown and crisp.
Meanwhile, coat a stove-top grill pan or griddle with cooking spray and set over medium-high heat to preheat. Season both sides of tuna steaks with salt and black pepper. Place tuna on hot pan and sear 2 to 3 minutes per side for medium doneness.
Meanwhile, in a small bowl, whisk together soy sauce, lemon juice, wasabi paste, and sesame oil. Transfer tuna to a serving plate and spoon dressing over top. Sprinkle the top with scallions. Serve wonton chips alongside.