- 3 pounds boneless duck breasts (3-4)
- Kosher salt, freshly ground pepper
- 2 tablespoons vegetable oil
- 1/4 cup Dijon mustard
- 2 tablespoons English mustard powder
- 2 teaspoons fresh lemon juice
- 5 tablespoons olive oil, divided
- 4 radishes, thinly sliced
- 4 small turnips, scrubbed, thinly sliced, plus 2 cups torn turnip greens or kale
- 6 cups torn mustard greens; plus any mustard flowers (optional)
- 1 tablespoon red wine vinegar
- Flaky sea salt
- Preheat oven to 400°. Score fat side of duck breasts 1/8″ deep in a crosshatch pattern; season both sides with kosher salt and pepper. Heat 1 tablespoon vegetable oil in a large ovenproof skillet, preferably cast iron, over medium. Cook 2 duck breasts, skin side down, until fat is rendered and surface is deeply browned and crisp, 10-15 minutes; transfer to a plate. Wipe out skillet and repeat with remaining duck and 1 tablespoon vegetable oil.
- Arrange all duck breasts in skillet, fat side up, and roast in oven until an instantread thermometer inserted into the center of breasts registers 135°, 5-8 minutes. Transfer to a cutting board and let rest at least 5 minutes and up to 2 hours.
- Whisk Dijon mustard, mustard powder, lemon juice, and 3 tablespoons olive oil in a small bowl; season mustard sauce with kosher salt and pepper.
- Toss radishes, turnips, greens, vinegar, and remaining 2 tablespoons olive oil in a large bowl; season with kosher salt and pepper.
- Thinly slice duck. Scatter greens over a platter (or two) and top with duck. Sprinkle with sea salt and serve with mustard sauce alongside.