Peel and slice potatoes to measure around 6 cups— for me this was 2.5 large purple potatoes.
Melt the ghee in a saucepan on the stove. Add the arrowroot powder and stir for a few minutes or until combined.
Pour in the almond milk and bring to a low boil, stirring often. When the sauce thickens, turn down the heat to medium and cook for about 10-15 minutes. Add salt, pepper, onion and garlic.
Prepare a casserole dish, brushing ghee on the sides and bottom. Layer the potatoes, overlapping.
Pour over the sauce and sprinkle with thyme. Cover with foil.
Bake at 350F for 30 minutes.
Uncover and bake for another 60 minutes.
Remove from oven and let stand for 10 minutes as the sauce thickens. Enjoy!