Preheat the oven to 350°. Set a 9 ¾ inch quiche dish on a half sheet pan.
To make the filling, add the broccoli florets to a pot and cover with water. Cover and bring the water to a boil. Cook until the florets turn bright green and are just al dente, about 5 minutes. Drain and add to a big mixing bowl.
Heat a cast-iron or nonstick skillet over low heat. Add the oil. When it is shimmering, add the leeks, garlic, and salt, and use a wooden spoon to coat the vegetables. Cook until the leeks are bright and translucent, and the garlic is fragrant, 5 to 7 minutes.
Transfer the leeks and garlic into the bowl of broccoli. Mix to combine, and transfer to the quiche pan. Spread the mixture out evenly.
To make the custard, add the water, cashews, tofu, yogurt, chickpea flour, oil, nutritional yeast, tamari, miso and turmeric to a blender. Blend until combined and creamy, 1 to 2 minutes depending on your machine.
Pour the custard over the broccoli and leeks, distributing it evenly. Bake for at least 1 hour, or until the top has browned and the center is firm to the touch.
As it cools, the top will deflate just a bit and the quiche will continue to firm up. Slice and serve.
This recipe contains nuts. You must discolse to anyone you serve this vegan frittata to that it contains nuts in case they are allergic.