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.