Preheat the oven to 425 degrees F (220 degrees C). Line a baking sheet with parchment paper.
Combine cauliflower, onion, and garlic in a bowl. Add olive oil, Parmesan cheese, and Cheddar cheese and stir to coat. Arrange on the prepared baking sheet.
Cook in the preheated oven until lightly browned, 35 to 45 minutes.