Instructions

Adjust oven rack to middle position and heat oven to 450 degrees. Spray rimmed baking sheet with vegetable oil spray. Whisk 2 tablespoons extra-virgin olive oil, 1 tablespoon whole-grain mustard, ½ teaspoon table salt, and ¼ teaspoon pepper together in bowl. Brush 1 head green cabbage, cut through core into 1¼-inch wedges, with oil mixture, then arrange cut side down on prepared sheet. Roast cabbage until beginning to brown around edges, about 12 minutes.
Remove sheet from oven and arrange 14 ounces kielbasa sausage, sliced ¾-inch thick on bias, cut side down on sheet around cabbage. Return sheet to oven and roast until cabbage is tender and kielbasa is browned, about 10 minutes. Transfer cabbage and kielbasa to serving platter; cover to keep warm.
Meanwhile, heat remaining 2 tablespoons oil in 12-inch nonstick skillet over medium heat until shimmering. Add 1 pound frozen pierogi (they will overlap) and cook until light golden brown, about 3 minutes per side. Carefully add ⅓ cup water, cover, and cook until pierogi are tender and water has been mostly absorbed, about 3 minutes. Uncover skillet and continue to cook until remaining water has evaporated, about 1 minute. Transfer pierogi to platter with cabbage and kielbasa. 
Melt 2 tablespoons unsalted butter in now-empty skillet over medium heat. Whisk in remaining 1 tablespoon water, remaining 1 tablespoon whole-grain mustard, remaining ⅛ teaspoon salt, and remaining pinch pepper. Drizzle cabbage, kielbasa, and pierogi with sauce and sprinkle with ¼ cup torn dill. Serve.