Duck Breast With Mustard Greens, Turnips, and Radishes

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.