Preheat the oven to 400 F and prepare a large, oval casserole dish (I used a 9x13") by greasing with coconut oil.

Scrub the sweet potatoes well and slice into thin rounds (about 1/8"-1/4" thick) either using a sharp knife, or a mandolin slicer. Arrange the sweet potatoes in a circular formation on the edge of the casserole dish, overlapping them as you go. Layer the remainder of the sweet potatoes in the center of the circle, in a straight line. Alternatively, you can layer the sweet potato slices in vertical rows if you're using a square or rectangular casserole dish. Set aside.

In a separate bowl, whisk together the coconut milk, arrowroot starch, nutritional yeast, sage, salt and pepper. Whisk well to throughly combine and set aside.

Using a medium stock pot, heat 1 tbsp of coconut oil on medium heat. Add the onion, shallots and garlic and cook for 3-4 minutes or until the onion is transluscent.

Pour the coconut milk mixture over the onion mixture and whisk well to combine. Simmer for 3-4 minutes to allow the mixture to thicken. Remove from heat.

Begin slowly pouring the coconut milk mixture over the sweet potatoes. Use a rubber spatula to spread the liquid between the sweet potatoes slices and evenly coat over the top until all of the mixture is used.

Bake in to the preheated oven for 35-40 minutes or until the sweet potatoes and cooked and the sauce has lightly browned.

Serve fresh with a serving spoon.