Preheat oven to 325° and grease a 13x9 pan with ghee or coconut oil. Set aside.
Wash potatoes well. Place a mandolin over a large bowl and slice potatoes. Stop before getting to close where you could slice your finger. Alternatively, you can use a food processor with a slicing blade.
Make the cream sauce: In a small saucepan combine the ghee and cassava flour until combined and thick. Cook 1 minute stirring continually. Add in the water, almond milk, and garlic oil and whisk until smooth and thick, about 3-5 minutes. Add in the salt, thyme, rosemary and pepper. Whisk again until well combined. Take off heat. Mixture should be thick, but pourable. Add a little water if it's too thick to pour over potatoes.
Add a thin layer of the cream sauce to the bottom of the pan and top with a layer of potatoes. You don't need to do single layers of potatoes, about 4 total layers will work. After the layer of potatoes add the sauce and spread as evenly as possible. Continue until all the potatoes have been used and top with the remaining sauce.
Cover with foil and bake 45 minutes then remove the foil and bake 40-45 minutes or until browned on top and potatoes are tender.
*If you don't need it low fodmap, you can use garlic powder in place of the garlic oil.