Potato Leek Au Gratin

Story by Holly
Potato Leek Au Gratin
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Say hello to your new favorite show-stopping holiday side dish, Potato Leek Au Gratin.

POTATOES. Leeks sauteed in butter, three kinds of cheese. Whole milk and heavy cream. POTATOES. It’s a fool-proof recipe for happy faces at the dinner table.

Julia Child said “if you’re afraid of butter, use cream” and this over-the-top take on potatoes au gratin with Gruyére wholeheartedly embraces that spirit of joie de vivre.

Potato Leek Au Gratin

Five Things to Love About This Recipe: 

  1. Butter.
  2. Make ahead! This leek and potato gratin is the ideal make-ahead dish, and we know those words are music to your ears. You can easily assemble this recipe 48 hours ahead of time. 
  3. Potatoes au gratin are pure indulgence! And we believe that indulgence deserves a place at your table, especially at times of celebration. The holiday season can feel hectic, but gathering around the table doesn’t have to. What if a shared holiday meal was a time of peace and simplicity? What if the easy goodness of potatoes au gratin really could set a tone of comfort and joy? 
  4. Leeks and potatoes are in season together! Many farmers markets begin to look pretty sparse late in the season, but leeks and potatoes are usually available even as the weather takes a turn for the crisp. Who doesn’t love an opportunity to cook with what’s in season? 
  5. Heavy cream. Also, again, butter. 

How to Make Potato and Leek au Gratin:

A few hints to make the going a bit easier when it comes to tackling this cheesy, leek-y riff on potatoes au gratin: 

  • Wash your leeks after you prep them. Leeks can trap a little grit in between their fine layers, so give them a good rinse under running water after you’ve chopped them to be sure they’re totally clean. 
  • On Gruyére: A Swiss cheese can only be labeled “Gruyére” if it is made in that region of Switzerland. Because it is aged for at least a year and then imported, it can be a bit more expensive than other Swiss cheese you might find. We think its distinctive rich, nutty flavor is worth the splurge, but you can substitute a less expensive Swiss cheese and your guests will be none the wiser. 
  • Selecting a blue cheese. Most any blue will work for this, but be aware that some are much more pungent than others. We like a relatively mild, crumbly blue for this dish—particularly if you’re feeding a crowd that may include some picky eaters. 
  • Resist the urge to swap out lower fat milk or cream. It’s a creamy, bubbly, buttery baking dish full of potatoes au gratin with Gruyére! It’s soul food, not health food. And that’s OK! Swapping out a lower fat dairy for the heavy cream and whole milk will result in a watery gratin, which is just no fun. Reserve this recipe for when you’re ready to live it up. 
Potato Leek Au Gratin

What Makes Potatoes “Au Gratin”? 

Essentially,  an “au gratin”—classically made with potatoes—is a dish that is covered with a layer of crusty, bubbly cheese. In his classic Oxford Companion to Food, Alan Davidson explains: 

“In the 16th century or beyond, the noun [gratin] referred to that part of a cooked dish which stuck to the pot or pan and had to be scraped (gratté) off if it was not to be wasted. Since the 19th century the meaning has changed to the effect deliberately created by cooks when they cook a dish so that it has a crisply baked top...often achieved by strewing grated cheese on top.”

Potato and leek dauphinoise—while similarly a baked, creamy dish of sliced potatoes and leeks with a bubbling, browned crust—is differentiated from a potato gratin like this by the fact that it potato dauphinoise is always made without cheese. Which seems like a missed opportunity if you ask us—we’re firmly in Camp Gratin. 

Brimming with butter and cream, these festively decadent leek-studded potatoes au gratin with plenty of Gruyére and blue cheese are a soul-soothing celebration of abundance.

Other Starchy Sides You’ll Love: 

Classic Potatoes Au Gratin is a simpler take on this creamy favorite. Or, if you’re looking for a slightly healthier option, our Healthy Sweet Potato Casserole is a colorful, autumnal treat. 

If you make this potato and leek au gratin, we’d love to hear about it! Be sure to snap a photo, add it to your Instagram feed or stories and tag us @themodernproper and #themodernproper if you do. Also, feel free to leave a comment on the post and tell your friends where you discovered the recipe.

Potato Leek Au Gratin

Potato Leek Au Gratin

Serves 6

Ingredients

4 tbsp butter
1 1/2 cups whole milk
1 cup heavy cream
3 large garlic cloves finely chopped
1 tbsp stone ground mustard
1/4 tsp freshly grated nutmeg
3/4 tbsp kosher salt
1 tsp freshly ground pepper
2 oz blue cheese
6 oz grated Gruyere cheese
2 oz freshly grated parmesan cheese
3 lb leek, white and light green portions, washed and sliced into 1/4 inch rings
2 lb russet potatoes (3-4 medium), peeled and cubed into 3/4 inch pieces
3 tbsp minced fresh chives

Method

  1. Position a rack in the center of the oven and heat the oven to 400°F. Grease a 9x13-inch baking dish or a large cast iron skillet with butter.
  2. In 12 inch nonstick skillet over medium heat, melt the butter. Add the leeks, stirring until leeks are completely coated. Cover and cook, stirring occasionally, until the leeks are tender, about 20 minutes. Add the milk, cream, blue cheese, garlic, mustard, nutmeg, salt and pepper and simmer uncovered. Stir the mixture occasionally, until slightly thickened, about 15 minutes. Be careful not to let boil. Remove from heat, and allow to cool.
  3. In a large bowl mix the gruyere and parmesan cheese.
  4. Layer one third of the potatoes in the baking dish or skillet. Pour one third of the leek mixture over the top. Sprinkle one third of grated cheese and 1 tbs on chives. Repeat to make two more layers reserving the last laying of chives.
  5. Cover Au Gratin with foil and bake for 45 minutes. Remove cover and continue baking until potatoes are completely tender when poked with fork and the crust is golden brown, about 30 more minutes. Allow to sit for 15 minutes after removing from oven. Garnish with reserved chives and serve.