Heat a large skillet or griddle to medium-high heat. Cook duck breast, fat side down, if not removed, for 10 to 12 minutes flipping with tongs, until it is lightly pink inside. Remove from skillet or griddle and place on cutting board, allowing meat to rest. While the duck is resting, begin the dressing.
In a medium bowl, mix fish sauce, lime juice, orange juice, red chile, grated ginger, and sesame oil. Stir using a spoon. Take baby spinach and lay across a decorative plate. Slice duck breast on a diagonal into thin slices. Place the juice from the cutting board and the duck slices into the dressing. Toss the duck slices in the dressing using your hands or tongs. Place the duck and dressing on top of spinach. Top with orange zest and serve immediately.