Carmelized Onion and Potato Gratin

By Patlynn

To ensure that the potato slices for our Caramelized Onion and Potato Gratin were all the same thickness, we used a mandoline instead of slicing them by hand. We caramelized the onions in a third of the time by adding brown sugar and cooking them over medium-high heat instead of low heat. Dark strings of caramelized onions were unappealing in our gratin, however. So instead of slicing, we chopped the onions and tucked them away in a middle layer of the gratin.

Ingredients

• Thinly sliced potatoes are key to an evenly cooked gratin—use a mandoline slicer or the slicing disk on a food processor. You’ll need 3 cups of chopped onions.
• 2 tablespoons unsalted butter
• 3 onions (see note), chopped
• 2 teaspoons salt
• 1 tablespoon brown sugar
• 1 cup low-sodium chicken broth
• 2/3 cup white wine
• 2/3 cup heavy cream
• 1 teaspoon balsamic vinegar
• 3 pounds russet potatoes, peeled and sliced 1/8 inch thick (see note)
• 2 teaspoons minced fresh thyme
• 1 teaspoon pepper
• 1 1/2 cups shredded Gruyère cheese

Preparation

Step 1

1. CARAMELIZE ONIONS Adjust oven rack to middle position and heat oven to 350 degrees. Melt butter in large nonstick skillet over medium-high heat. Add onions and 1 teaspoon salt and cook until just beginning to brown, about 5 minutes. Stir in brown sugar and cook, stirring frequently, until onions are golden brown, about 5 minutes. Add ¹⁄₃ cup broth and cook until softened and deep golden brown and sticky, about 5 minutes. Transfer caramelized onions to medium bowl.


2. DEGLAZE PAN Bring remaining broth, wine, cream, and remaining salt to boil in now-empty skillet, scraping up any browned bits. Off heat, stir in vinegar; cover and keep warm.


3. ASSEMBLE GRATIN Shingle half of potatoes in 3-quart baking dish; sprinkle with 1 teaspoon thyme and ½ teaspoon pepper. Spread caramelized onions over potatoes and top with remaining potatoes, remaining thyme, and remaining pepper. Pour broth mixture over potatoes.


4. ADD CHEESE Bake until bubbling around edges, about 30 minutes. Top with Gruyère and bake until gratin is golden brown and fork inserted into center meets little resistance, 30 to 40 minutes. Let cool 15 minutes. Serve.


MAKE AHEAD: Caramelized onions and sauce can be refrigerated in separate airtight containers for 1 day. To finish, microwave onions and sauce separately for 30 seconds. Proceed with step 3.



Quicker Caramelization


It usually takes about 45 minutes to caramelize onions. But we got it done in about 15.


BARELY BROWN
The onions begin to brown after they've cooked in butter for five minutes at medium-high heat.



SUGAR AID
Cook the onions for an additional five minutes with a tablespoon of brown sugar to intensify the browning.



READY TO GO
Simmer the onions in a little broth for a final five minutes to prevent scorching.