Preheat the oven to 400 degrees. Line a baking sheet with aluminum foil and coat with non-stick spray.
Whisk together the mustard, maple syrup, and soy sauce. Pat the salmon dry and place skin side down on the baking sheet. Season with salt and pepper and brush with half of the syrup mixture.
Bake for 18 to 20 minutes.
Meanwhile, simmer the remaining mixture over low heat, reduce by half.
Brush with the remaining soy sauce mixture and allow to cook for 5 minutes longer.
Serve over oven roasted mapled Brussels sprouts