Set oven to 350˚F (176˚C)
Boil cashews on high for 20 min and drain and rinse. Put them in the blender.
Slice potatoes on a mandolin as thinly as possible and cover with cold water in a large bowl to prevent browning.
Sauté Vidalia onion in 1 tablespoon olive oil for approximately 7 minutes, or until softened.
Remove the onions from the pan and put in the blender.
Add the 2 cups of plant-based milk, 1 teaspoon paprika, ¼ cup nutritional yeast, 1 tablespoon white wine vinegar, 1 tablespoon lemon juice, and salt and pepper to taste to the blender and blend on the highest speed until smooth and creamy. Scrape the sides once or twice while blending to ensure that there are not solids left.
Place the 3 tablespoon of olive oil in a medium skillet over medium heat.
Add the 3 tablespoon of flour and continuously whisk until it forms a thick paste and lightly browns.
Pour the cashew milk mixture into the pan and whisk until the roux thickens into a smooth and creamy sauce, approximately 3 minutes.
Evenly pour a thin layer of the cashew cream into the bottom of a deep 8 x 8 inch baking dish
Form the gratin by layering sliced potatoes and the cashew cream. For example, place a layer of potatoes over the sauce, then add a layer of sauce, add another layer of potatoes, and so on until you run out of potatoes. Pour the remaining cashew cream over the top and cover the dish with foil and bake for an hour.
Uncover the dish and bake for an additional 20 minutes, or until it has developed a crust on top and is a light golden brown.
Garnish with the minced thyme.