Combine shallots and garlic. Rub 1 teaspoon shallot mixture over each fillet; cover and chill. Place remaining shallot mixture, broth, juice, turnips, and wine in a small saucepan. Bring to a boil; cook until reduced to 1 1/4 cups (about 10 minutes). Stir in mustard and cornstarch; bring to a boil. Reduce heat; cook 1 minute or until thick. Keep warm.
Steam turnip leaves, covered, 30 seconds or until wilted. Rinse the turnip leaves with cold water, and drain well. Pat dry with paper towels. Remove stems; discard. Sprinkle fillets with salt and pepper. Place 1 fillet in center of 2 turnip leaves. Fold in 4 sides of leaves to cover fillet, forming a packet. Repeat procedure with the remaining fillets and leaves. Steam packets, seam sides down, covered, 7 minutes or until fish is done. Drizzle sauce evenly over the packets. Garnish with lemon slices, if desired.