Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish.
Slice potatoes into thin, uniform rounds (about 1/8 inch thick).
In a saucepan, combine cream, milk, minced garlic, and butter. Heat until butter melts, but don't let it boil.
Layer 1/3 of the potato slices in the baking dish, slightly overlapping them.
Sprinkle with salt, pepper, diced onions, and 1/3 of the cheddar cheese.
Repeat layers twice more with remaining ingredients.
Pour the warm cream mixture evenly over the potatoes.
Top with Parmesan cheese and fresh thyme.
Cover with foil and bake for 45 minutes.
Remove foil and bake for an additional 15-20 minutes until golden brown and bubbly.