Peel and thinly slice the potatoes. I use a mandoline slicer with a 2.5mm blade.
Submerge your potato slices in a bowl of cold water to prevent oxidation.
Preheat the oven to 425°F.
In a medium pot, whisk together the non-dairy milk, flour, nutritional yeast, vegetable bouillon, garlic powder, onion powder, thyme, paprika, pepper, and cayenne.
Bring the sauce mixture to a simmer on medium heat.
Reduce heat to medium-low and let simmer for 5 minutes, whisking frequently. Taste test: Does it need more salt?
Drain the potatoes and layer half of them in a casserole dish. Pour half of the sauce over the potatoes. Give the casserole dish a good shake so the sauce gets in the nooks and crannies of the potatoes.
Do the same with the remaining potatoes and sauce.
Cover the dish with foil and bake for 25 minutes. Uncover and bake for an additional 15-20 minutes, or until the potatoes are fork tender.
Let sit for 5-10 minutes before serving. The sauce will thicken a bit more as it sits.
Notes
This recipe was updated December 2020. For those who loved it, the previous version called for 2 cups of vegan milk and 1 cup of vegetable stock. It also called for cornstarch as a thickener. To use cornstarch, simmer all sauce ingredients together (except for the flour). In a small bowl, whisk together 1/4 cup of cold water and 1/4 cup cornstarch. Whisk into the simmering sauce. Remove from heat when thickened. Everything else in the recipe is basically the same.