Preheat the oven to 450 degrees F. Line a baking sheet with parchment paper and set aside.
In a large bowl, combine the cauliflower florets, onion, celery, mushrooms, olive oil, garlic powder, Italian seasoning, salt, and pepper. Stir the mixture until all of the vegetables are well coated in the seasonings.
Spread the mixture in a single layer on the sheet pan, using two pans if necessary.
Roast for 30 minutes, turning halfway, until the cauliflower is golden brown and the onions are caramelized.
Top the stuffing with the fresh parsley before serving.