Preheat the oven to 425°F (220°C).
Grate the cauliflower head and place it in a microwave-safe bowl. Microwave for about 8 minutes or until soft.
Using a clean kitchen towel, squeeze out as much water as possible from the cooked cauliflower.
In a large bowl, combine the cauliflower, mozzarella cheese, Parmesan cheese, beaten egg, oregano, basil, salt, and black pepper. Mix until well combined.
On a parchment-lined baking sheet, press the cauliflower mixture into a pizza crust shape.
Bake the crust for 15-20 minutes or until it becomes golden brown and crispy.
Remove the crust from the oven and spread tomato sauce over it. Top with bell peppers, red onion, and spinach.
Bake for an additional 5-10 minutes or until the veggies are tender.
Slice and serve hot. Enjoy your healthy cauliflower pizza!