Preheat your oven to 425 degrees while you prepare the crust.
Peel the sweet potatoes and slice into 1/4” thick pieces to form the “crust”. With olive oil, lightly grease the slices.
Arrange the slices, slightly overlapping, in the bottom of a 9” deep dish pie plate. The potatoes will shrink while cooking, so do overlap. Cut several slices into half rounds and arrange along the sides, overlapping to form the side crust.
Season lightly with sea salt and pepper and roast in the preheated oven for 10 minutes.
While the crust roasts, toss the Brussels Sprouts with 1 tbsp olive oil, sprinkle with salt and arrange in a single layer on a baking sheet.
After the crust roasts for 10 minutes, add the Sprouts to the oven and roast for an additional 15-20 minutes, or until the sprouts are soft and beginning to get crispy and the crust begins to brown. Once roasting is complete remove pans and, lower the oven temperature to 400 degrees.
While the crust and sprouts roast, heat a medium-sized skillet over medium heat and add 1 tbsp olive oil.
Add the chopped onions and stir to coat. Once onions are soft and translucent, add the sausage to the pan and stir to break up the sausage. Add the pinch of red pepper flakes if desired.
In a large mixing bowl, whisk together the eggs, coconut milk, nutritional yeast, and salt.
Layer the Brussels sprouts and then the sausage mixture over the butternut squash in the pie dish. (If opting for dairy option, add a layer of shredded cheese to the top.) Pour the egg mixture over all the layered ingredients. This mixture will flow through any gaps in the sweet potatoes, helping to form the crust when baking.
Bake at 400 degrees for 25-30 minutes, until center is set and puffing up and the edges begin to brown.
Remove from the oven and allow to cool for 10 minutes before slicing and serving.