Preheat oven to 400°F (200°C). Line baking sheet with parchment.
Pulse cauliflower in food processor until rice-like. Steam for 5 minutes, then cool slightly.
Wrap cauliflower in cheesecloth, squeeze out excess moisture completely.
In a bowl, mix cauliflower, almond flour, flax eggs, garlic powder, oregano, salt, and pepper.
Shape mixture into a round crust on parchment-lined sheet. Bake 25 minutes until firm and golden.
Spread marinara sauce evenly, sprinkle vegan mozzarella, and arrange bell pepper, mushrooms, onion, and spinach.
Bake 10 more minutes until cheese melts. Garnish with fresh basil before slicing.