1. Preheat the oven to 400 degrees . Place salmon skin side down on a rimmed baking sheet lined with foil. Combine 1/2 cup mustard, 2 tablespoons maple syrup and 1/4 teaspoon pepper and spread evenly over top of salmon. Bake until just opaque in the center, 20 to 22 minutes. Carefully separate salmon from skin, discard skin and cut salmon into 6 pieces.
2. While salmon is roasting, thinly slice brussels sprouts or shred in a food processor or with a box grater; transfer to a large bowl. Whisk together oil, lemon juice, salt, and remaining mustard, maple syrup and pepper. Toss brussels sprouts with dressing and nuts and serve with salmon.