Pre-heat oven to 400° Lightly grease a 9" oven-safe baking dish with the coconut oil.
In a saucepan, over medium heat combine coconut milk, rosemary, garlic, thyme., salt & pepper. Bring to a boil, whisking.
Make a slurry by combining the cornstarch with ¼ cup cold water and whisk until smooth, or shake in a small jar with a lid.
Whisk the cornstarch slurry into the coconut milk, bringing back to a boil, continuing to whisk until smooth and thickened. Remove from heat and remove rosemary sprig.
In the prepared baking dish, layer sweet potato slices, then onion slices, continuing until done.
Pour the garlic & herb-infused coconut milk evenly over the casserole.
cover with foil, and allow to bake 60 minutes
remove foil and bake another 15 minutes.
Remove from oven and allow to rest 10-15 minutes before serving.
Notes
Full fat coconut milk works best for this recipe. Use both the coconut cream and the coconut water, which will be separated in the can. The two will combine easily as they are heated and whisked.