In a shallow bowl, whisk buttermilk and Dijon until smooth. In another bowl, combine the cornmeal and seasonings. Dip fillets into buttermilk mixture, then into cornmeal mixture. Place 1 inch apart on a wire rack coated with cooking spray. Place rack on a baking sheet. Broil 4 inches from the heat for 3-4 minutes on each side or until fish flakes easily with a fork. Serve with lemon if desired.