Preheat your oven to 450 degrees F. Line a large rimmed baking sheet with high-heat-resistant parchment paper or nonstick foil.
In a large bowl, stir together the chopped cauliflower, onion, celery, and garlic. Toss with the melted butter, 1 teaspoon kosher salt, black pepper, thyme, and sage.
Spread the cauliflower mixture on the prepared baking sheet. Roast for 15 minutes.
Remove the pan from the oven. Add the chopped mushrooms, stirring them into the mixture. Sprinkle with the remaining teaspoon of salt and lightly spray with olive oil.
Return the pan to the oven and roast until the stuffing is browned, 10-15 more minutes.*
Garnish the dish with chopped parsley and serve.
Video
Notes
*To promote browning, if your pan is broiler-safe, you can roast the veggies for just 10 minutes, then broil them, 6 inches below the heating element (not directly below), for 2-3 more minutes.