Preheat oven to 375ºF.

Prepare the potatoes:

• Scrub the potatoes and thinly slice them into 1/8'' rounds on a mandoline. Transfer to a bowl of cold water and set aside.

Prepare the baking dish:

• Butter a 9x12 oval baking dish. Set aside.

Cook the mushrooms and leeks:

• Melt 3 tablespoons of butter in a wide pot over medium-high heat. Add the mushrooms and leeks and cook for 8–12 minutes until the mushrooms turn golden brown—season with salt and pepper and transfer to a bowl.

Prepare the cream sauce:

• Melt the 3 tablespoons butter into the same pot and add the flour. Cook, stirring the flour into the butter, for 1–3 minutes until nutty and fragrant. Add ½ cup of cream and whisk regularly to incorporate. Continue adding the cream in ½ cup increments, whisking after each addition, until all of the cream is added and the sauce is smooth. Cook over medium heat for an additional 3–4 minutes until thickened slightly. Season with salt, nutmeg, and white pepper. Turn off the heat.

Assemble the gratin:

• Arrange a layer of potatoes–overlapping them slightly–on the bottom of the prepared baking dish. Pour a third of the sauce over the top and use the back of your spoon to smooth it out to ensure even coverage. Next, add a third of the cheese.
• Arrange another layer of overlapping potatoes and add half of the remaining sauce. Scatter half of the mushrooms on top and add half of the remaining cheese. If your gratin appears to mound in the middle, use your hands to flatten it out (it happens to the best of us).
• Arrange the final layer of potatoes on top. Pour the remaining sauce on top. Note, if you're short on sauce, supplement with a few splashes of cream on top! Add the remaining cheese and then finish with the remaining mushrooms and leeks. Smooth it out.

Bake the gratin:

• Transfer the baking dish to the oven and bake for 45 minutes or until the potatoes are fork-tender and the sauce is bubbly and browned on top.

Prepare the breadcrumbs:

• Melt the remaining 1 tablespoon butter into a skillet. Cut the challah into large cubes and add it to the skillet. Cook, occasionally turning, for 3 minutes until golden brown all over. Turn off the heat.
• Transfer the bread cubes to a food processor with the parsley and garlic and pulse until finely minced. Set aside.

Broil the gratin:

• Remove the gratin from the oven and turn on the broiler. Scatter the breadcrumbs on top and transfer to the broiler for 1-3 minutes until golden brown on top. Turn off the heat.

To serve:

• Let the gratin stand at room temperature for 10 minutes. Serve warm. Enjoy!