Steam or microwave the riced cauliflower until tender, about 6-7 minutes. Let cool slightly.
Place the cooked cauliflower in a clean kitchen towel or cheesecloth and squeeze out as much moisture as possible. This is crucial for a crispy crust.
In a bowl, combine the squeezed cauliflower, salt, garlic powder, Italian seasoning, Parmesan cheese, egg, and 1/2 cup of the mozzarella cheese. Mix well.
Transfer the cauliflower mixture to the prepared baking sheet and shape it into a pizza crust about 12 inches in diameter.
Bake in a 400°F oven for 20-25 minutes, or until the crust is golden brown and set.
While the crust is baking, heat the olive oil in a skillet over medium heat. Add the mushrooms and cook until softened and lightly browned, about 6-7 minutes.
Add the kale and spinach to the skillet and cook until wilted, about 3-5 minutes. Remove from heat.