Scoop seeds out of cucumber halves and discard. Cut cucumber into 1-inch strips.
Thinly slice white parts of green onions. Slice green parts into 1-inch pieces.
Heat sesame oil in a pan over high heat. Sprinkle five-spice powder and salt over sliced duck; add to the pan. Stir-fry for 1 minute. Add sliced white onion and fry for 1 minute. Add cucumber, green onion pieces, and hoisin sauce; stir-fry for 1 minute. Remove pan from heat.
Place lettuce and cabbage leaves on a plate and fill with duck mixture. Garnish with sesame seeds. Serve hot.
