Preheat the oven to 400°F. Line a half sheet pan with foil.

In a medium bowl, toss the cabbage, onion, 2 tablespoons oil, ¾ teaspoon salt, and ½ teaspoon pepper until well combined. Place on the prepared baking sheet.

In the same bowl, toss the frozen pierogies with 2 tablespoons oil, ½ teaspoon salt, and ¼ teaspoon pepper until the pierogies are well coated. Arrange on the prepared baking sheet along with the sliced kielbasa.

Bake for 20 minutes. Stir the cabbage and flip the pierogies. Bake until the cabbage is golden and the kielbasa and pierogies are crispy, 20 to 30 minutes more.

Meanwhile, in a medium bowl or measuring cup, whisk together the vinegar, mustard, honey, garlic, remaining ¼ teaspoon salt, and ¼ teaspoon pepper. Whisk in the remaining ¼ cup oil until well combined. Drizzle half of the dressing over the cabbage, pierogies, and kielbasa, tossing to combine. Top with dill or parsley and serve hot with the remaining dressing and sour cream.