Place walnuts in food processor; coarsely chop. Add breadcrumbs, lemon rind, olive oil and dill; pulse until crumbly. Mixture should stick together. Season with salt and pepper; set aside.
Arrange salmon fillets skin side down on parchment paper lined baking sheets. Brush tops with mustard.
Spoon 1/3 cup of walnut crumb mixture over each fillet; gently press the crumb mixture into the surface of the fish. Cover with plastic wrap; refrigerate for up to 2 hours.
Bake at 350°F 15 to 20 minutes, or until salmon flakes with a fork. Just before serving, sprinkle each with 1 teaspoon lemon juice.