Sweet Potato-Potato Gratin with Caramelized Leeks, Spinach, and a Pecan Crumb Topping

I love the mingling of sweet potatoes and potatoes in this gratin — and the surprise of the green spinach and leeks peeking out beneath the orange and white. Those sweet leeks and a hint of garlic lend a depth of flavor to this dish, which also gets a boost from a mix of cheeses and a savory toasted pecan and breadcrumb topping. This takes a bit of time to prep, but once you get it in the oven, you’re freed up to do other things.

As a Thanksgiving side dish, this would be enough for six servings. If you want more than that, I would double the recipe but fill two separate baking dishes rather than one larger. The smaller versions cook more evenly.  This recipe was adapted from one I wrote for my cookbook The Fresh and Green Table.

Serves 6 as a side dish

• 2 tablespoons butter, plus more for rubbing dish
• 1 cup fresh bread crumbs
• 1/3 cup finely chopped toasted pecans
• 2 tablespoons extra virgin olive oil
• 1 tablespoon chopped fresh thyme
• Kosher salt
• 2 cups thinly sliced leeks (white and light green parts from about 2 large leeks, about 6 ounces), well washed
• 1 ½ teaspoons minced fresh garlic
• 4 ounces  (about 4 cups packed) baby spinach leaves
• 12 ounces sweet potato 
• 8 ounces russet potato 
• 3/4  cup heavy cream
• 3/4 cup chicken broth, homemade vegetable broth, or water
• ¾ cup coarsely grated sharp cheddar cheese, swiss cheese, or gruyere  (or a combination)
• ¼ cup coarsely grated Parmigianno Regianno
• 1 teaspoon Dijon mustard
• Freshly ground black pepper

1. Heat the oven to 350 degrees F. Rub a 2-quart shallow gratin dish or other baking dish (an 11 x 7 Pyrex is fine) with a little butter. 

2. In a small bowl, combine the breadcrumbs, the pecans, 1 tablespoon olive oil, 1 teaspoon thyme, and a big pinch of salt. Stir well to combine and set aside.

3. In a 10-inch heavy nonstick skillet, melt 2 tablespoons of the butter with 1 tablespoon of the olive oil over medium-low heat. Add the leeks and a big pinch of salt, cover, and cook, stirring occasionally, until softened and translucent, about five minutes. Uncover, raise the heat to medium, and continue to cook, stirring frequently, until the leeks are shrunken and browned in places (some more than others), about eight minutes more. Add the minced garlic and stir until softened and fragrant, about 30 seconds. Add all of the spinach to the pan and toss with the leeks until the spinach is completely wilted, about 1 minute.

4. Transfer the leeks and spinach to the gratin dish and arrange in one even layer by smoothing out with a silicone spatula. (It will be a thin layer.)

5. Peel the potato and sweet potato. Cut them in half lengthwise and turn the halves cut-side down on the cutting board. Then, using a very sharp knife (a Santoku is good for this), cut them cross-wise into very thin, half-moon shaped slices. (It’s more important to get the potatoes thin than perfect. Try for slices about 1/16-inch thick.)

6. Put the potatoes and the sweet potatoes into a mixing bowl. Add the cream, the broth or water, the cheese, the remaining thyme, the Dijon, 3/4 teaspoon salt and several grinds of fresh pepper.  Mix well.

7. Transfer all the contents of the bowl to the gratin dish. Use a spatula (and your hands!) to distribute the vegetables as evenly as possible. Then use your palms to press down on the veggies to bring the liquids up and around them as much as possible. (It won’t necessarily completely cover them.)

8. Cover the top evenly with the breadcrumb mixture.

9. Bake until the veggies are tender when pierced with a paring knife or fork (check the middle of the dish as well as the sides), the breadcrumbs are brown, and the juices around the edges of the gratin have bubbled down and formed a dark brown rim around the edge, 60 to 65 minutes.

10. Let cool for 15 to 20 minutes before serving.