Preheat the oven to 400°F (204°C). Spray a 13 x 9–inch (33 x 23–cm) baking dish with nonstick cooking spray.

In a medium saucepan over medium heat, melt the butter. Add the onion and sauté, reducing the heat as needed to prevent burning, for 8 to 10 minutes, or until the onion is very soft and translucent.

While the onion is sautéing, drain the soaked cashews and add them to a blender. Add the broth, milk, nutritional yeast, mustard, salt and black pepper. Blend on high for 3 to 5 minutes, scraping down the sides of the blender if needed, until the sauce is completely smooth. Set the sauce aside.

Add the flour to the onion and stir to coat the onion in the flour and form a paste.

Add the sauce to the onion-flour mixture and whisk to combine. Remove the saucepan from the heat and whisk the sauce constantly for 1 to 2 minutes, until it is thick and smooth.

Place the hash browns in a large bowl and season them with salt and black pepper. Pour the sauce on top of the hash browns. Toss to fully combine.

Scoop the mixture into the prepared baking dish. Smooth out the hash browns evenly.

Bake the casserole for 30 minutes. Remove the casserole from the oven.

In a small bowl, combine the cornflakes and oil.

Top the casserole evenly with the cornflake mixture, and bake for 10 to 15 minutes, or until the topping is brown and crispy.

Top the casserole with the scallions and serve.