Preheat the oven to 400°F. Line a half sheet pan with foil.
In a medium bowl, toss the cabbage, onion, 4 tablespoons olive oil, 1 1/4 tsp salt, garlic and onion powder and pepper until well combined. Place on the prepared baking sheet.
Arrange on the prepared baking sheet along with the sliced kielbasa.
Bake for 20 minutes. Stir the cabbage. Bake until the cabbage is golden and the kielbasa is crispy, 20 to 30 minutes more.
Meanwhile, in a medium bowl or measuring cup, whisk together the vinegar, mustard, keto-friendly sweetener, garlic, remaining ¼ teaspoon salt, and ¼ teaspoon pepper. Whisk in the remaining ¼ cup olive oil until well combined. Drizzle half of the dressing over the cabbage, and kielbasa, tossing to combine. Top with dill or parsley and serve hot with the remaining dressing and sour cream.