Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
Rice the cauliflower by pulsing it in a food processor or grating it with a box grater until it resembles rice.
Place the riced cauliflower in a microwave-safe bowl and microwave for 4-5 minutes, or until soft. Allow it to cool slightly.
Transfer the cauliflower to a clean kitchen towel and wring out as much moisture as possible, which is crucial for a crispy crust.
In a mixing bowl, combine the drained cauliflower, egg, mozzarella, Parmesan, Italian seasoning, garlic powder, salt, and pepper. Mix well to form a dough.
Spread the cauliflower mixture onto the prepared baking sheet, shaping it into a round pizza base about 1/4 inch thick.
Bake the crust for 15 minutes, or until golden and firm to the touch.
While the crust bakes, sauté the chopped spinach in a pan over medium heat until wilted, then set aside.
Once the crust is done, spread the cream cheese over it as the base sauce.
Top with sautéed spinach, crumbled feta, and shredded mozzarella cheese. Sprinkle with red pepper flakes if using.
Return the pizza to the oven and bake for an additional 10-15 minutes, or until the cheese is bubbly and slightly golden.
Remove from the oven, let cool for a few minutes, and then slice and serve.