Preheat the oven to 400°F (200°C). Line a baking sheet (or pizza pan) with parchment paper.
Make a flax egg: mix the ground flaxseed and water in a small bowl. Let stand until thick and gel-like, about 5 minutes.
Rice the cauliflower: remove leaves and core, chop into florets, then pulse in a food processor until it looks like rice (avoid over-processing into mush).
Cook the cauliflower rice in a microwave-safe bowl for about 8 minutes (or until tender). Cool for 5–10 minutes, then transfer to a clean towel or cheesecloth.
Squeeze out moisture: wring the towel firmly to remove as much water as possible. The drier the cauliflower, the crispier and sturdier the crust.
In a mixing bowl, combine the squeezed cauliflower with the flax egg, almond flour, chickpea flour, garlic powder, oregano, and salt. Mix until it forms a cohesive dough.
Form the crust: press the dough onto the prepared sheet into a round about 1/4-inch thick, smoothing the edges.
Bake the crust for 25 minutes, until the edges are golden. (For extra crispness, bake 5 minutes longer.)
Add toppings: spread the tomato sauce over the crust, then scatter sliced cherry tomatoes (and optional vegan mozzarella/parmesan).
Return to the oven for 10 minutes, just until warmed through. Top with fresh basil, slice, and serve warm.