Smash the roasted garlic in the bottom of a large bowl. Stir in raw garlic and fresh thyme. Sprinkle with a healthy pinch of salt and pepper, then stir to mix. (And maybe another healthy pinch of salt. Potatoes love salt.)
Stir in 2/3 of the cheeses and the heavy cream. Stir to combine.
Wash and peel the potatoes. Thinly slice on a mandolin, and immediately toss in the creamy cheese mixture. Thinly slice the onions, and toss with the creamy potatoes. Coat all sides of the potatoes and onions in creamy cheese if possible.
Grease a small casserole dish with butter. (Mine is a Staub 11″, 2 quart baking dish.) Create a stack of of potatoes and onions, and place them on their sides into the prepared dish. Continue adding onto the sideways stack of potatoes and onions in the dish until completely full.
Sprinkle the remaining cheese over the top of the dish.
Cover the dish with foil, and place it onto a rimmed baking sheet (in case any liquids bake over the top of the dish). Cook for one hour at 375 F. After an hour, uncover the dish and bake for an additional 30 minutes until golden brown and crispy.
Let cool for about 10 minutes before serving. Enjoy!
Notes
*Shred all cheeses from a block of cheese if possible. Pre-shredded cheese contains anti-caking agents that prevent it from melting as evenly.