Preheat oven to 425°F. Line a large rimmed baking sheet with parchment paper.
Cut cabbage lengthwise into 6 (¾-inch) slices; arrange on the prepared baking sheet.
Combine 2½ tablespoons oil, 1 teaspoon each garlic powder and Italian seasoning, ½ teaspoon salt and ¼ teaspoon pepper in a small bowl. Brush the cabbage evenly with the oil mixture. Roast until tender, 20 to 25 minutes.
Meanwhile, combine 2 tablespoons vinegar and 1 tablespoon mustard in a small bowl.
Remove the cabbage from the oven and flip the steaks. Brush evenly with the vinegar mixture. Roast until lightly browned, about 5 minutes more. Serve sprinkled with parsley, if desired.
