Heat oven to 425°F and position a rack in the center.
Line a large baking sheet with parchment for easier cleanup.
Toss cauliflower with olive oil, garlic powder, onion powder, paprika, salt, and pepper on the sheet.
Roast 15 minutes, stir, then continue roasting 10 minutes until tender with browned edges.
Combine cheddar, Parmesan, panko, and melted butter in a bowl until evenly moistened.
Sprinkle the cheesy crumb mixture over cauliflower and roast 3–5 minutes until melted and golden.
Broil 30–60 seconds for extra color if desired. Garnish with parsley and serve hot.