Drizzle butter and 2 tablespoons oil into a 13x9-in. baking dish; tilt to coat bottom. Place salmon in dish; brush with remaining oil and sprinkle with salt and pepper.
Bake, uncovered, at 425° until fish just begins to flake easily with a fork, 18-22 minutes. Meanwhile, combine the mayonnaise, lemon juice and wasabi. Sprinkle salmon with onions and sesame seeds. Serve with sauce.