Whisk together soy sauce, sliced scallions, canola oil, sesame oil, toasted sesame seeds, grated ginger, 1 tablespoon water, and grated garlic in a medium bowl. Pour 1/2 cup soy sauce mixture into a large ziplock plastic bag. Add salmon cubes, press out excess air, and seal; refrigerate at least 30 minutes or up to 1 hour. Reserve remaining soy sauce mixture; set aside.
Preheat oven to broil with rack 8 inches from heat. Remove salmon from marinade; arrange in a single layer on a large rimmed baking sheet lightly coated with cooking spray. Discard marinade. Broil salmon until lightly charred and nearly cooked through, about 7 minutes.
Meanwhile, cook grattini in a large saucepan of boiling salted water until al dente, 4 to 5 minutes; drain well. Divide pasta evenly among 4 bowls. Top evenly with salmon, avocado, cucumber, chile, and cilantro. Serve with reserved soy sauce mixture.