How to Make It
In a large bowl, mix 2 tbsp. each soy sauce and rice vinegar, the ginger, garlic, sesame oil, and Chinese five spice. Add duck breasts, turn to coat well, cover, and chill 2 hours. Lift breasts from marinade (discard marinade), and lay, skin up, on a baking sheet; chill, uncovered, overnight.
In a small bowl, mix hoisin, sugar, mustard, and remaining 2 tbsp. each soy sauce and rice vinegar. Fill another bowl with ice water, and add green onions.
Preheat oven to 425°. Set a large ovenproof frying pan over medium-high heat. When hot, add vegetable oil, then duck breasts, skin down. Cook until skin is browned and crisp, about 5 minutes. Carefully tilt pan and spoon out excess duck fat. Turn breasts skin up, and transfer pan to oven.
Bake until duck is still slightly pink in the center of thickest part (cut one to test), about 8 minutes. Let stand for about 5 minutes. Drain green onions.
Transfer duck breasts to a board and thinly slice. Carefully separate pancakes and lay each flat. Fold each round in half and add several slices of duck. Spoon a little sauce over duck, and top with slivers of green onion. Fold one side of pancake over duck, then roll up into a bundle. Serve with remaining dipping sauce.
*You can find moo shu pancakes (sometimes labeled "moc shu") in Asian markets and in well-stocked supermarkets.