Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Slice the purple cabbage into 1-inch thick wedges and arrange them on one side of the baking sheet.
Season the chicken breast evenly on both sides with garlic powder, dried oregano, sea salt, and black pepper.
Place the seasoned chicken breast on the other side of the baking sheet.
Drizzle half of the olive oil over the cabbage wedges and the chicken, tossing the cabbage gently to ensure it is lightly coated.
Roast for 20 to 25 minutes until the chicken reaches an internal temperature of 165°F and the cabbage edges are caramelized and tender.
While roasting, whisk together the remaining olive oil, lemon juice, and finely chopped fresh parsley in a small bowl.
Slice the roasted chicken and serve it over the cabbage wedges, then finish the dish with a sprinkle of pumpkin seeds and a drizzle of the vinaigrette.