If you’re using dried Porcini mushrooms, put them in a small bowl and cover with about a cup of boiling water. Let stand for about 45 minutes. Then drain, reserving the liquid, to use in place of the stock. Cut the mushrooms into small pieces.

Preheat oven to 375°F.

Butter a 9x13x2-inch baking dish.

Put the cream, bay leaf, salt, stock or mushroom liquid, and liquid smoke, if using, in a medium pan and stir to combine. Bring the mixture to a simmer, reduce heat to low and let steep while you prepare the potatoes.

Peel the potatoes, then cut into 1/8 inch thick slices. Work quickly, but safely, so the potatoes don’t oxidize and turn brown. Do not put the potatoes in water; it will wash away the starches that make the gratin creamy.

Place the potatoes in the baking dish, overlapping slightly, so it looks kind of like roof shingles. If you are using mushrooms, sprinkle pieces between each layer of potatoes.

Remove the bay leaf, and gently pour the cream over the potatoes.

Dot with butter. Just cut the butter into small pieces and place each piece on top of the potatoes, trying to get them somewhat evenly distributed. Cover tightly with foil.

Bake covered until the potatoes are almost tender, about 35 minutes. You should be able to pierce the potatoes moderately easily with a small sharp knife.

Increase the temperature to 400°F. Remove the foil, press the potatoes down with a spatula into an even thickness, allowing the creamy juices to baste the potatoes. Sprinkle the thyme and black pepper on top. Continue to bake, pressing the potatoes down periodically, until the gratin is nicely browned, about 30 minutes.

Let stand 5 minutes. Remove the gratin from the oven. It’ll be a little loose and creamy at this point; that’s okay, it’ll thicken a bit more while standing.