9x12 Oval Baking Dish
- 2 pounds Yukon gold potatoes
- 7 tablespoons butter divided
- 1 pound mixed mushrooms such as chestnut, baby bella, maitake, shiitake, or oyster; torn or sliced depending on the variety
- 2 small leeks white parts only, trimmed, rinsed, and thinly sliced
- 1 tablespoon flour
- 1¾ cup heavy cream plus more as needed
- ½ teaspoon nutmeg
- ¼ teaspoon white pepper
- ½ cup grated cheese like parmesan, manchego, or gruyère
- ¼ pound challah or brioche bread
- 2 teaspoons dry parsley or 1 tablespoon freshly minced
- 2 teaspoons garlic powder
- Salt and pepper to taste
Cook the mushrooms and leeks:
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.
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.
Calories: 450kcal | Carbohydrates: 33g | Protein: 8g | Fat: 33g | Sodium: 217mg | Fiber: 4g | Sugar: 3g | Vitamin C: 23mg