How to Make It
In a food processor, pulse the cauliflower until finely chopped (you should have about 4 cups). Transfer to a microwavable bowl and toss with 2 teaspoons of salt and 1/2 teaspoon of pepper. Cover with plastic wrap and microwave at high power until tender, about 5 minutes. Scrape the cauliflower onto a clean kitchen towel and let cool until no longer steaming, about 10 minutes. Wrap the cauliflower in the towel and squeeze until nearly dry, then transfer to a large bowl. Add the almond flour, eggs and 1/2 cup of the Parmesan; stir with a rubber spatula until well combined.
Preheat the oven to 400°. Line a baking sheet with parchment paper and lightly coat it with nonstick cooking spray. Scrape the cauliflower mixture onto the prepared baking sheet and pat it into a 10-inch round, about 1/2 inch thick. Bake the cauliflower crust for about 15 minutes, until it springs back lightly to the touch and the edge is golden brown. Remove from the oven and let cool slightly.
Increase the oven temperature to 450°. In a medium bowl, combine the remaining 1/4 cup of Parmesan with the mozzarella and Fontina. Sprinkle about two-thirds of the cheese mixture on the crust, leaving a 1/2-inch border. Top the cheese with the mushrooms, onion, chile, parsley and rosemary. Sprinkle the remaining cheese over the vegetables, drizzle with olive oil and season with salt and pepper. Bake the pizza until the vegetables are softened and the cheese is lightly browned and bubbling, about 15 minutes. Cut into wedges and serve.