Place cream. thyme, sage and orange peel in a sauce pan. Bring to a boil and turn off heat. Allow mixture to sit for at least 15 minutes allowing flavors infuse into cream. Strain mixture and set infused cream aside.
Peel and slice sweet potatoes into 1/8" slices.
Spray a casserole dish (3 1/2 quart size) with non-stick cooking spray.
Layer 1/3 of the sweet potato slices in the bottom of the casserole dish, overlapping slices.
Sprinkle with salt and pepper.
Pour 1/3 of the infused cream over potatoes.
Repeat 2 times until all sweet potatoes and cream are used.
After final layer, press sweet potatoes down firmly in pan.
Cover with foil and place on a baking sheet. Place in a preheated 350 degree oven for one hour. Remove foil and continue baking for 45 minutes.
When sweet potatoes are cooked trough and the mixture is bubbly and browned, sprinkle on Parmesan cheese. Return to oven for 15 minutes.
Allow casserole to rest for 10-15 minutes before serving.