4scallionswhite and light green parts only, chopped
Whisk together all ingredients in a small bowl and set aside.
Preheat oven to 475F. Melt butter and 2 tbsp sesame oil in large glass baking dish inside oven as it warms.
Place salmon in butter/oil mixture, skin-side down if skin is on. Brush fillet with remaining 1 tbsp of sesame oil. Roast for 6-12 minutes depending on the thickness of the fillet. Salmon will be opaque and flake easily with a fork when done. Be sure to watch carefully to avoid overcooking it.
Sprinkle salmon with salt and pepper, toasted sesame seeds and scallions.
Place a dollop of wasabi mayonnaise on each serving of fish and serve immediately. Pass around any remaining mayonnaise at the table.