Preheat oven to 450F. Cut four 12x16-inch sheets of parchment paper. Fold in half; crease fold and unfold.
On half of each sheet of parchment, place 1/2 cup leek and top with a sprig of thyme. In a bowl stir together mustard, wine, and white pepper. Add scallops and shrimp; toss to coat. Spoon one-fourth of seafood mixture over leek. Top with 1/4 cup carrot and 1/4 cup zucchini. Drizzle with 1 tablespoon butter and top with a lemon slice. Fold parchment over seafood stack like a book. Start sealing at the edge of the fold. Working in 2-inch sections, fold about 1/4-inch of the open edges over and crease tightly. Fold again. Continue working around the open edges of the parchment, overlapping folded sections slightly. Finish with a double fold at the end of the packet. Make sure folds are creased tightly so they don't open. Repeat to make three more packets. Place packets in a very large baking sheet. Bake in the preheated oven for 10 to 12 minutes or until scallops and shrimp are opaque. To serve, cut an "X" in the top of the parchment; serve immediately. Makes 4 servings.