Preheat the oven to 350 degrees. Spray an 11x7-inch baking dish with nonstick cooking spray and place salmon in it.
In a food processor or blender, combine the remaining ingredients except the butter and process until the onions are in crumbs (about 5 seconds). Place 1/4 cup of the onion mixture on each fillet and drizzle with the melted butter. Bake for 20 minutes or until the fish flakes easily with a fork.