01 - Preheat oven to 375°F (190°C). Lightly grease a medium casserole dish with butter or nonstick spray.
02 - If using whole cauliflower, cut into florets and pulse in a food processor until the texture resembles rice grains.
03 - Melt butter in a large skillet over medium heat. Add diced onion and cook for 3 minutes until translucent. Stir in minced garlic and sauté for 1 additional minute.
04 - Add riced cauliflower to the skillet and cook for 5 to 6 minutes, stirring frequently, until softened and moisture has mostly evaporated.
05 - Add chopped spinach to the skillet and sauté until wilted, about 2 minutes.
06 - Reduce heat to low. Stir in heavy cream, cream cheese, 3/4 cup Parmesan, 1/2 cup mozzarella, salt, pepper, nutmeg, and Italian herbs. Mix gently until the sauce is creamy and ingredients are well incorporated.
07 - Spoon the mixture evenly into the prepared casserole dish. Sprinkle the remaining mozzarella and Parmesan cheese over the surface.
08 - Bake for 20 to 25 minutes, or until the casserole is bubbling and the cheese topping turns golden brown.
09 - Remove casserole from oven and let cool for 5 minutes. Garnish with chopped fresh parsley before serving.