Potatoes Au Gratin

Mashed potatoes might be the go-to holiday side dish, but if you really want to impress your guests, you’ve got to make these potatoes au gratin. Infused with garlic and thyme and layered with Gruyère and Parmesan, a bubbling dish of these rich and creamy potatoes will truly stop the show.

Potatoes au gratin sounds fancy and looks impressive, but it’s actually very simple to make. It takes little time, but most of this is hands-off waiting in which you can be doing other tasks. And if you don’t have that time on the day you want to actually serve it, you can even make it in advance (see below for more on that!). Now let’s get into the specifics on how to make this classic and classy side.

How To Make Potatoes Au Gratin

INGREDIENTS

• Yukon Gold Potatoes: While many recipes call for the more starchy russet potato, I prefer Yukon Golds. They have a medium starch content that still provides plenty of thickening power—and they offer a creamy buttery texture that just takes this dish over the top.
• Heavy Cream & Milk: I find that a 1:1 ratio of heavy cream and milk provides the perfect creamy base for this gratin. If you have half-and-half, you can just use that!
• Shallot: The sweeter baby sister of onions, shallot is perfect for infusing a mild onion vibe into the milk and cream.
• Garlic: Crushed garlic will infuse better into the cream than whole garlic cloves, while still remaining easy to strain out.
• Thyme: Fresh is definitely the way to go here. If you only have dried, I’d recommend leaving it out of the steeped milk and cream mixture and only sprinkling 1/4 tsp. between each gratin layer.
• Kosher Salt: I developed this recipe using Diamond Crystal kosher salt. If you have Morton or need to use table salt, I highly recommend cutting the salt quantity in half, since these salts are denser by volume than Diamond Crystal kosher salt.
• Gruyère: Cheese is traditional in potatoes au gratin (in fact, it’s what differentiates it from scalloped potatoes). With its rich nuttiness and slight funk, Gruyère brings the perfect balance to all the rich creaminess.
• Parmesan: A little Parmesan adds pops of salt and acidity to balance out the Gruyère. Fresh grated from a block is always best!

STEP-BY-STEP INSTRUCTIONS

In a medium pot, combine the quartered shallot, crushed garlic, whole thyme sprigs, and some cream and milk. Cover and set the pot over medium-high heat. Bring to a simmer, stirring occasionally to make sure the bottom isn’t scorching. Once the mixture is simmering, remove the pan from the heat, cover, and let it sit untouched for 30 minutes so the aromatics can infuse into the cream and milk.

In the meantime, butter a 13" x 9" baking dish and preheat the oven to 350° with the rack in the middle position. While the cream and milk are infusing, prep the potatoes by peeling them and slicing into 1/8" slices. I prefer using a small handheld mandoline for this: it’s the best tool for getting super-even, super-thin slices. Mandolines can be intimidating for some people, but as long as you use the guard, you should be just fine. There’s no need to be concerned about the potatoes oxidizing here: One huge perk of Yukon Gold potatoes is that they don’t oxidize like russets do. Simply transfer the potatoes to a bowl and keep uncovered at room temperature until ready to use.

Once the cream and milk are done infusing, strain the mixture through a fine mesh strainer into a bowl (you can discard the shallot, garlic, and thyme stalks). Return the cream mixture to the pot and add the salt and the potatoes. Bring everything to a simmer over medium-high heat, then cook, stirring occasionally, until the potato slices are fork-tender, which should take 4 to 5 minutes. This simmering step kick-starts the cooking of the potatoes, but it also activates their starch, drawing it out and using it to thicken the sauce. By the end of the 5 minutes, you will notice the cream mixture has thickened considerably.

Transfer a third of the potatoes and cream mixture to the prepared baking dish, then sprinkle with a third each of the Gruyère, Parmesan, and fresh thyme leaves. Repeat these layers once, then finish by topping with the remaining potatoes, Gruyère, and Parmesan. Hold on the thyme on the top for now—you’ll sprinkle it over the top after it bakes.

Bake the gratin, uncovered, for 40 to 50 minutes, or until it’s bubbling and golden brown on top. Wait about 5 minutes before topping with thyme and serving!

The full list of ingredients and instructions can be found in the recipe below.

Recipe Tips

• Use a mandoline: A mandoline is your best friend when it comes to slicing potatoes for gratin. It cuts the prep time in half, and delivers perfectly sliced potatoes—which, in addition to looking nicer, will also cook more evenly. Many people are skittish about mandolines, but as long as you use the guard, you should be just fine.
• Don’t be afraid of the salt: A whole tablespoon might seem like a ton, but it’s not a misprint. Potatoes absolutely DRINK salt, and the richness of the heavy cream and whole milk demand it too. Please note though, that if you’re using table salt or Morton kosher (which are saltier by volume), you should decrease the salt by about half.
• Leave some pretty potatoes for the top: For the prettiest potato gratin, pick out some super gorgeous potato pieces and set them aside for the top layer; otherwise you’ll end up using the potatoes at the bottom of the pot for the very top of your gratin—and often, they’re not the prettiest.

Make Ahead & Storage

If you’d like to make these potatoes in advance, cook the entire dish all the way through, then let it cool completely before covering and refrigerating it for up to 3 days. When you’d like to serve, reheat them, covered, in a 350° oven. Smaller leftover portions can be stored in an airtight container and reheated in the microwave.