Preheat the oven to 400°F (200°C). Grease 2 rimmed baking sheets or line them with parchment paper.

To make the flatbread pizza, mix the gluten-free flour, corn flour, arrowroot flour, garlic powder, salt, and pepper together in a bowl. Add the oil and, while mixing, gradually incorporate just enough water to achieve a thin pancake batter–like consistency, starting with 1½ cups (360 ml) and adding up to 1 cup (240 ml) more, a few tablespoons at a time.

Spread the batter onto the prepared baking sheets, creating two 10- to 12-inch (25 to 30 cm) pizza crusts. Bake for 20 minutes, until the edges are light golden brown. Remove from the oven and set aside. Increase the oven temperature to 430°F (220°C).

To make the onion-blueberry topping, while the crust bakes, add the oil to a large pan and sauté the onions over medium-high heat for 15 to 20 minutes, until caramelized, stirring frequently and adjusting the temperature as needed to prevent burning. Deglaze with a splash of water whenever the onions begin to stick and scrape the bottom of the pan to reincorporate any browned bits.

Add the coconut milk, reduce the heat, and let simmer for about 10 minutes, or until it has the consistency of a thick, spreadable sauce. Season with salt and pepper.

Spread the onion mixture evenly over the pizzas, leaving a perimeter around the edges for the crust. Sprinkle the blueberries evenly over the top. Bake the pizzas for 10 minutes, or until the blueberries are slightly softened and the crust is deep golden brown.

Garnish with fresh basil leaves, an extra drizzle of olive oil, and a sprinkle of freshly ground black pepper. Enjoy straight from the oven or cold!