Whether you’re team mashed potato or team potato gratin, this easy mushroom and potato leek gratin recipe is a Thanksgiving potato recipe everyone will love.
If you’re looking for an exquisite Thanksgiving potato recipe, you’ve come to the right spot. This mushroom and potato gratin recipe is aromatic, earthy, and oh so creamy and rich.
Normally, I’d place myself squarely on the team mashed potato side of the imaginary line. I’ve always found mashed potatoes to be completely unassailable. However, this potato leek gratin recipe really has me questioning my alliance.
The best part of this gratin? The buttery, homemade herbed breadcrumbs. You’ll make these as the gratin bakes in the oven. Once the gratin has finished baking, you’ll sprinkle the herbed breadcrumbs on top and then broil until they turn crispy and brown.
The only downside to gratin? The prepwork! But the finished dish looks beautiful, tastes delicious, and is worth the extra work as far as I’m concerned!
How to make this mushroom and potato leek gratin recipe:
First, start with large, golden potatoes. You can peel them, but I prefer to the leave the skin on the potatoes. Use a mandoline (preferred) or food processor with slicing attachment or a very sharp knife and slice the potatoes into 1/8” slices. The mandoline is the fastest way to achieve this, so if you don’t have one I recommend picking one up! It’s a really versatile tool that can be used for so many things! Keep the potatoes in water to prevent them from browning.
Next, cook the mushroom and leeks. Simply sauté both in butter until the mushrooms release their liquid and the liquid evaporates and they turn golden brown. Be careful not to burn the leeks! Transfer to a bowl.
In the same pot, add more butter and a tablespoon of flour. Cook until the flower is nutty and aromatic, about 2 minutes. Slowly whisk in heavy cream, about a quarter cup at a time, until smooth and thick. Simmer for 5–7 minutes until thickened. The secret to this cream sauce? Nutmeg and white pepper. Don’t skip on them! They add just slightly sweet, lightly spiced, and very aromatic undertone to the gratin.
How to assemble the gratin:
Now it’s time for assembly! Butter a 12×9 inch baking dish (oval looks nice with the round potato shapes). Arrange a layer of potatoes, slightly overlapping. Pour in a third of the cream sauce. Add another layer of potatoes, a third more of the sauce, and then half of the mushrooms. As you layer, you will sprinkle grated cheese on top. Add the final layer of potatoes and then the remaining sauce and the rest of the mushrooms. Bake for 40 minutes.
Depending on the size and dimensions of your baking dish, you may have more layers or fewer layers. It really does not matter how you arrange the mushrooms, but you want a layer or two of mushrooms in the middle of the casserole and you want enough remaining to scatter on the top, so as long as you keep that in mind, you should be good!
While the casserole bakes, prepare the homemade breadcrumbs and scatter them on top. Broil for 1–2 minutes until golden brown.
Let stand for 10–15 minutes before serving.
Looking for more Thanksgiving side dish recipes? Check my archives!
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Mushroom and Potato Leek Gratin
- 9×12 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
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 9×12 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.
- Let the gratin stand at room temperature for 10 minutes. Serve warm. Enjoy!