Start by soaking the quinoa. Cover the quinoa with at least 1" of water. Ideally, soak for 6 - 8 hours. If you're in a hurry, use very hot water and soak for at least 30 minutes.
Once you're ready to make the crust, preheat the oven to 425ºF. Line a baking pan with parchment paper (either rectangle or square). Spray the entire surface with cooking spray.
Thoroughly rinse the soaked quinoa, then add to a blender. Add the 1/4 cup of water, baking powder, salt, and olive oil. Blend on high until smooth and creamy. The batter should resemble a thick pancake batter.
Pour batter onto the prepared pan. Spread it out into an even layer, about 1/8" thick (again, either in a circular pan like 9" cake pan OR into a rectangle on a traditional cookie sheet).
Bake for 20 minutes, then remove the quinoa pizza crust, flip it directly onto the baking pan (flip and remove the parchment paper from the top side). Return the pizza to the oven baking for another 10 - 15 minutes until it's starting to brown and the edges are crispy.
Remove and top with sauce, cheese and any other toppings you'd like. Bake for another 12 - 15 minutes until cheese has melted.
Remove and let cool for a few minutes in the pan, transfer cutting board, and slice. Garnish with herbs, pepper flakes, grated cheese, etc., and serve immediately!