01 - Set oven to 375°F. Lightly grease a medium casserole dish with butter.
02 - If using fresh cauliflower, pulse florets in a food processor until they resemble rice grains.
03 - Melt butter in a large skillet over medium heat. Add diced onion and cook for 3 minutes, stirring, until translucent. Add minced garlic and sauté for 1 minute.
04 - Add cauliflower rice to the skillet. Stir frequently and cook for 5 to 6 minutes, until slightly softened and excess moisture has evaporated.
05 - Add chopped spinach and cook for about 2 minutes, until wilted.
06 - Lower the heat to medium-low. Stir in heavy cream, cream cheese, 3/4 cup Parmesan, 1/2 cup mozzarella, salt, pepper, nutmeg, and Italian herbs. Mix continuously until smooth and creamy.
07 - Transfer the mixture into the prepared casserole dish. Sprinkle with the remaining mozzarella and Parmesan cheeses.
08 - Bake uncovered for 20 to 25 minutes, until the top is bubbly and golden brown.
09 - Remove from oven and let cool for 5 minutes. Garnish with chopped parsley before serving.