Preheat oven to 400 degrees Fahrenheit and grease a 9x13 inch baking dish with butter.
In a medium saucepan, combine heavy cream, minced garlic, thyme, salt, and pepper over medium heat until the mixture reaches a simmer.
Arrange half of the sweet potato slices in an even layer in the prepared baking dish.
Pour half of the heated cream mixture over the potato layer.
Add the remaining sweet potato slices in a second even layer and pour the remaining cream mixture over the top.
Cover the dish tightly with aluminum foil and bake for 30 minutes.
Remove the foil, sprinkle the grated parmesan cheese over the top, and return to the oven for 15 minutes or until potatoes are tender and the top is golden.
Allow the dish to rest for 10 minutes at room temperature to set the sauce before serving.