Preheat oven to 425 degrees F.
1. Season duck with five-spice powder, salt, and pepper. Place duck skin side up on baking sheet and bake for 20 minutes or until an instant-read thermometer inserted into the thickest part of the duck reads 130°. Transfer duck to a cutting board; let rest 10 minutes.
2. Meanwhile in a large pot, bring 7 cups of water to a boil. Remove and discard the bottom third of the broccolini stems. If some stems are very thick, cut them in half lengthwise. Add broccolini to boiling water and cook over high heat for 2-3 minutes, or until the stalks are crisp yet tender. Drain in a colander and run cold water over broccolini. Set aside.
3. Heat the oil in a large skillet. Add the garlic and cook over medium heat, stirring occasionally until garlic is translucent and aromatic. Add the broccolini, sea salt and red pepper flakes. Stir until heated through. Serve with duck legs.