Preheat your oven to 400 °F (200 °C) and line a baking sheet with parchment paper.
Add the halved radishes to a large bowl. Drizzle with olive oil, then sprinkle with garlic powder, dried parsley, salt, and pepper. Toss until evenly coated.
Spread the seasoned radishes in a single layer on the baking sheet, cut‑side down for best browning.
Roast in the preheated oven for 30–35 minutes, stirring once about halfway through, until the radishes are tender inside with golden, slightly crisp edges.
Remove from the oven and immediately sprinkle with grated Parmesan. Let the heat melt the cheese slightly before serving.