Preheat the oven to 400°F (205°C) and line a baking sheet with baking paper.
If you are using fresh cauliflower, steam it until tender. If using frozen cauliflower, let it completely thaw. Transfer the cauliflower to a food processor and process until it reaches a rice-like consistency.
Transfer the cauliflower into a clean dishtowel or cheesecloth and squeeze out all the excess water. Ensure you remove as much moisture as possible.
In a mixing bowl, add the cauliflower rice, mozzarella cheese, egg, smoked paprika, and oregano. Mix until well combined into a dough.
Transfer the cauliflower dough onto the lined baking sheet and shape it into a pizza crust. Bake in the oven for 25 minutes until the crust turns slightly golden. You can also bake the crust for 20 minutes, flip it over and bake for another 5 minutes for an extra-crispy crust.
Remove the crust from the oven and add your desired toppings. Add the keto pizza sauce, tomatoes, mushrooms, and oregano. Bake for a further 5 minutes until the tomatoes and mushrooms are soft.
Remove the pizza from the oven and sprinkle over the mozzarella cheese. Bake for another 5 minutes until the cheese is bubbly and golden.
Add fresh baby spinach to the pizza, slice, and serve hot.