Preheat the oven to 425°F (220°C). Line a 17-inch round or a 18x26-inch (45x66-cm) rectangular metal tray with parchment paper.
In a small bowl, mix together the flaxseed and water. Let sit for 5 minutes.
In a large bowl, combine the mashed sweet potato, almond flour, flax egg, salt, pepper, dried rosemary, and additional flaxseed. The mixture will be thick and pasty.
Spread the mixture onto the tray and flatten slightly. Place the sweet potato crust in the oven and bake for 30-35 minutes, until the edges are crisp and golden.
While the crust is baking, prepare the pesto. Add cashews, pistachios, fresh basil, cilantro, spinach, lemon juice, salt, pepper, garlic cloves, and olive oil to a food processor and process until mostly smooth with a bit of texture.
When the pizza crust is done baking, spread the pesto on top, then cover with bell peppers, cherry tomatoes, and red onions.
Return the pizza to the oven and bake for 5 minutes more. Allow to cool for 10-15 minutes.