From Alison Roman at Epicurious
3 pounds boneless duck breasts (3–4)
Kosher salt and freshly ground pepper
2 tablespoons vegetable oil
¼ 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
2 cups kale
6 cups torn mustard greens
1 tablespoon red wine vinegar
Flaky sea salt
Preheat oven to 400°. Score fat side of duck breasts ⅛" deep in a crosshatch pattern; season both sides with kosher salt and pepper. Heat 1 Tbsp. 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 Tbsp. vegetable oil.
Arrange all duck breasts in skillet, fat side up, and roast in oven until an instant-read 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 Tbsp. olive oil in a small bowl; season mustard sauce with kosher salt and pepper.
Toss radishes, turnips, kale, vinegar, and remaining 2 Tbsp. 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.