Preheat the oven to 350F.
Place cream, half and half, roasted garlic, nutmeg, onion powder in a sauce pot. Bring up to about a simmer, then remove from heat and cool.
Add chives, thyme, garlic, and eggs to cooled cream mixture.
Very thinly slice the potatoes on a mandolin.
Layer the potatoes in a 13 x 9 baking pan sprayed with cooking spray. Ladle egg mixture over the top of each layer, season with salt and pepper, and top with a handful of parmesan cheese. Continue layering in this fashion (until you run out of either custard or potatoes).
Spray the top with cooking spray and cover with foil. Bake for an hour and a half, then uncover and cook until top is browned and the dish has set, about another 20 minutes.