Preheat oven to 400°.
Heat oil in a large nonstick skillet over medium-high heat. Add leek; sauté 4 minutes. Stir in salt and pepper. Remove the mixture from pan, and set aside. Add endive to pan. Sauté 9 minutes; stir occasionally. Add garlic, and cook 1 minute. Remove from heat; stir in the leek mixture. Spoon into a 13 x 9-inch baking dish coated with cooking spray.
Combine wine, clam juice, and 2 tablespoons mustard in skillet; cook over medium-high heat until reduced to 2/3 cup (about 5 minutes). Remove from heat; stir in marjoram.
Spread 2 tablespoons of mustard evenly over both sides of fillets. Arrange the fillets on top of vegetables, and pour wine mixture over the fillets. Cover and bake at 400° for 20 minutes or until fish flakes easily when tested with a fork.