Preheat the oven to 400°F (200°C). Line a large rimmed baking sheet with parchment paper. Remove damaged outer cabbage leaves, rinse if needed and dry well. Trim only the dry end of the stem, leaving the core in place. Set the cabbage stem-side down and cut from top to bottom into slices about ¾ inch thick.
Arrange the cabbage slices on the baking sheet without overlapping. Brush both cut sides with the olive oil. Leave space around each steak; use a second pan if needed.
Mix the garlic powder, onion powder, smoked paprika, kosher salt and black pepper. Sprinkle evenly over both sides of the cabbage slices.
Roast for 20 minutes, until the edges start to brown and the centers begin to soften. Check smaller end pieces, which may cook faster.
Sprinkle the finely grated Parmesan evenly over the tops of the partly roasted cabbage steaks. Keep the cheese in a thin layer.
Return to the oven for 10 to 15 minutes, until the cheese is golden and the thick centers are tender when pierced. If the cheese browns before the cabbage softens, loosely cover the tops with foil and continue roasting until tender.
Lift the steaks from the pan with a wide spatula and serve warm, while the edges are crisp. Add the optional chopped parsley if desired.