Preheat your oven to 400°F / 200°C. Lightly grease a 1. 5-quart soufflé dish or casserole.
In a medium saucepan, par-boil the potato slices in lightly salted water for 3 minutes. They will not be fully tender.
Drain well and set aside.
While the potatoes cook, heat 1 tablespoon (15 ml / 0.5 fl oz) of oil in a large skillet over medium heat.
Add the mushrooms, leeks, garlic, rosemary, and salt.
Sauté, stirring occasionally, until the leeks are tender.
Stir the heavy cream into the skillet.
Bring the mixture to a boil, then immediately reduce the heat and simmer gently for about 30 seconds, or until the sauce has slightly thickened.
To assemble, arrange a third of the potato slices in the bottom of your prepared dish, overlapping them as needed. Spoon one-third of the leek mixture over the potatoes and sprinkle with 1/4 cup (960 ml) of the Parmesan cheese.
Repeat the layers once more.
Top with the remaining potatoes and the final third of the leek mixture.
Drizzle with the remaining 1 tablespoon (15 ml / 0.5 fl oz) of oil.
Bake, uncovered, for 30 minutes. Carefully remove from the oven and sprinkle the remaining 1/4 cup (960 ml) of Parmesan cheese over the top. Return to the oven and bake for an additional 5 to 10 minutes, until the potatoes are fork-tender and the top is golden and bubbly.
Let the gratin stand for 10 minutes before serving to allow it to set.
Serve warm, with a dollop of sour cream if you like.
