The Onions -
sauté onions and grated garlic in skillet over medium heat, with a pat of butter and a drizzle of olive oil.
Once onions are sweated (7-10 minutes), remove for heat and transfer to a bowl.
The Potatoes -
preheat the oven to 400°F. Grease ovenproof casserole dish with butter or non-stick spray.
Arrange a layer of sliced potatoes over the base, then a layer of the sautéed onions, with a light sprinkling of thyme leaves (no stems), parmesan cheese, and a tiny dash of salt and pepper. Continue this layering pattern until the casserole dish is topped off.
Gently pour the stock over the potatoes. Season the top layer with a little salt and pepper, then scatter the whole sage leaves and the rest of the parmesan cheese.
Disperse little pats of the butter all over the potatoes, then place the dish on the top third of the oven, and cook for 60-90 minutes, until the top golden crisp and the potatoes are cooked all the way through. Finish with a little black truffle salt if you happen to have some.