1. Preheat oven to 425 degrees Fahrenheit (225 degrees Celsius). Line a baking sheet with parchment paper and spray with cooking spray.
2. Lay out cauliflower and mushrooms on the same sheet and spray well with cooking spray. Season with salt, pepper, onion salt, and garlic powder.
3. Roast for 40 minutes or until the cauliflower is browned and there is little to no liquid from the mushrooms remaining.
4. Toss with fresh parsley and serve.