Soak the cashews in water for 4-6 hours, then drain.
In a blender, combine the soaked cashews, nutritional yeast, garlic, vegetable broth, salt, and pepper. Blend until smooth and creamy.
Preheat your oven to 350°F (175°C).
Slice the potatoes and butternut squash into thin pieces. Toss them in olive oil and spread them out on a baking sheet.
Roast the vegetables in the preheated oven for about 30 minutes, or until they are tender and slightly browned.
While the vegetables are roasting, sauté the onion until it is soft and translucent.
In an oven-safe pan, layer the sautéed onions, roasted potatoes, and butternut squash. Pour the cashew sauce over the top.
Increase the oven temperature to 375°F (190°C) and bake the casserole for about 35 minutes.
For the last 2 minutes of baking, switch the oven to broil to give the top of the casserole a golden color.
Garnish with fresh parsley before serving. Enjoy your delicious Vegan Roasted Potatoes and Butternut Squash Casserole!