One of the things I love most about the holiday season is all of the recipe testing that goes into the lead-up to The Big Day. Eating Thanksgiving dinner once is one thing, but getting to eat it for 3-4 weeks in a row?! Glorious. Just glorious. Another added bonus to this is that I can try out some of the more wild recipes that my family will never let me bring instead of the traditional favorites. For instance, I’m pretty sure there would be an uproar if I even so much as hinted that we go without mashed potatoes. However, making this potato, caramelized onion and leek gratin for a random Sunday dinner? Totally acceptable. Celebrated, even.
In all honesty, I’ve never been much for potato gratins. In my experience, they never cook evenly or all the way through so you end with mushy potatoes on top and potato bricks on the bottom. Apparently the key to potato casserole nirvana is very thinly sliced and very homogeneously sliced potatoes. You could get this result either by risking your fingertips by slicing them on a mandoline or you can use the slicing attachment for your food processor, which is what I did.
In this Potato, Caramelized Onion & Leek Gratin, the potatoes are layered with rich caramelized onions and leeks, along with a white wine-based cream sauce. It’s baked until thick and bubbly and then topped with a thick Manchego cheese layer. The resulting casserole is creamy without being overly so, with just the right amount of cold weather comfort. I’m not saying it will definitely kick mashed potatoes off of your family’s Thanksgiving line-up, but it does stand a fighting chance.
- 2 tablespoons unsalted butter
- 2 large onions chopped
- 1 large leek white and light green parts only, thinly sliced
- 2 teaspoons salt divided
- 1 tablespoon brown sugar
- 1 cup vegetable broth divided
- 2/3 cup white wine
- 2/3 cup heavy or light cream
- 1 teaspoon balsamic vinegar
- 3 pounds russet potatoes peeled and sliced 1/8 inch thick
- 2 teaspoons minced fresh thyme divided
- 1 teaspoon black pepper divided
- 1 1/2 cups shredded Manchego cheese
- Preheat oven to 350ºF.
- Melt the butter in a large nonstick or cast iron skillet over medium-high heat. Add the onions, leek, and 1 teaspoon of salt. Cook until just beginning to brown, about 5 minutes, stirring occasionally. Stir in the brown sugar and cook, stirring frequently, until the onions are golden, about 5 minutes. Stir in 1/2 cup of the broth and cook until softened and sticky, about 5 minutes more. Transfer to a medium bowl.
- Add the remaining broth, white wine, and heavy cream to the pan along with the remaining 1 teaspoon of salt. Scrape the bottom of the pan to release any browned bits. Bring to a boil and then remove from the heat. Stir in the vinegar.
- In a 3-quart baking dish, scatter half of the potatoes. Sprinkle with 1 teaspoon of the thyme and 1/2 teaspoon of the black pepper. Spread the caramelized onions over the potatoes. Top with the remaining potatoes, thyme, and black pepper. Pour broth mixture over the potatoes.
- Bake until bubbling, about 30 minutes. Top with the Manchego cheese and bake until golden brown and cooked through, about 30-40 minutes. Let cool for 15 minutes before serving.
This will turn out best if the potatoes are very thinly and evenly sliced. In order to achieve this, use a mandoline or the slicing attachment on a food processor. Gruyere cheese can be substituted for the Manchego, if desired.