Leek and Onion Scalloped Potatoes Recipe

Emily MorganPosted on February 21, 2026

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I published this recipe for Leek and Onion Scalloped Potatoes a few years ago after a small kitchen disaster turned into my favorite trick. I was making a classic version for a holiday dinner and realized I was completely out of regular onions. All I had were a couple of beautiful, pale green leeks staring back at me from the fridge. It’s a great reminder that some of the best comfort food, like this simple crockpot chicken with potatoes and green beans, often comes from improvisation.

I decided to just go for it. I sliced them thin and layered them in with the potatoes. What came out of the oven was a revelation. The leeks brought this incredible, mild onion flavor that was sweet and gentle, not sharp. It felt instantly more elegant. That happy accident is now my go-to method.

This dish became my secret weapon for turning a simple potato bake into something special. It’s the side I bring to spring gatherings and fancy dinners alike. Everyone always asks for the recipe, and I love telling the story of how it was born from a moment of “what do I do now?!”

Recipe Overview

Cuisine: American/French-Inspired
Category: Side Dish
Prep Time: 25 minutes
Cook Time: 1 hour 15 minutes
Total Time: 1 hour 40 minutes
Servings: 6-8 people

Why This Recipe is So Special

It’s all about the layers and the two types of alliums. Using both leeks and yellow onions creates a depth of flavor that’s hard to beat. The onions give a robust, savory base.

The leeks melt into a soft, sweet whisper throughout the creamy sauce. I also love a technique I picked up from an old cookbook: making a simple roux right in the saucepan after cooking the veggies.

This little step thickens the cream sauce perfectly. It keeps everything cohesive and luxurious without being gloppy. It’s a game-changer for texture, much like the creamy, cheesy sauce in these Cheesy Ranch Potatoes with Smoked Sausage.

The Full Ingredient List

Gathering your mise en place makes this process so smooth. Here’s everything you’ll need to make this beautiful gratin.

3 tablespoons unsalted butter
2 large leeks, white and light green parts only
1 medium yellow onion
3 cloves garlic, minced
3 tablespoons all-purpose flour
2 cups whole milk
1 cup heavy cream
1 teaspoon fresh thyme leaves (or 1/2 tsp dried)
1 teaspoon kosher salt, plus more for layers
1/2 teaspoon freshly ground black pepper
1/4 teaspoon freshly grated nutmeg
3 pounds Yukon Gold potatoes
1 1/2 cups freshly grated Gruyère cheese, divided

My Step-by-Step Method

Don’t let the layers intimidate you! We’ll build this dish step by step. Just take your time and enjoy the process.

Preheat your oven to 375°F (190°C). Butter a 9×13 inch or similar 3-quart baking dish.
Prepare your veggies. Halve the leeks lengthwise, rinse very well under water to remove grit, and slice thinly. Thinly slice the yellow onion. Peel the potatoes and slice them into 1/8-inch thick rounds.
Melt the butter in a large saucepan or Dutch oven over medium heat. Add the sliced leeks and onion. Cook for about 10 minutes, stirring often, until they are very soft and sweet. Do not let them brown.
Stir in the minced garlic and cook for 1 more minute until fragrant.
Sprinkle the flour over the cooked vegetables. Stir constantly for 2 minutes to cook the raw flour taste out.
Slowly pour in the milk and cream while whisking constantly. Bring the mixture to a gentle simmer. It will thicken noticeably.
Remove the sauce from the heat. Stir in the thyme, salt, pepper, nutmeg, and 1 cup of the grated Gruyère cheese until the cheese is melted.
Arrange half of the potato slices in the bottom of your prepared dish, overlapping slightly. Season lightly with a pinch of salt and pepper.
Pour half of the creamy leek and onion sauce over the first potato layer.
Repeat with the remaining potatoes and sauce. Press the top layer down gently so it’s submerged.
Cover the dish tightly with aluminum foil. Bake for 45 minutes.
Remove the foil. Sprinkle the remaining 1/2 cup of cheese over the top. Bake, uncovered, for another 25-35 minutes. The top should be golden brown and the potatoes should be tender when pierced with a knife.
Let the dish rest for 15 minutes before serving. This wait is crucial for the sauce to set!

My Top Tips for Success

Clean Leeks Thoroughly: Grit is the enemy! After slicing, I often swish the leek pieces in a big bowl of water to let the sand sink to the bottom.
Slice Consistently: Use a sharp knife or mandoline for even potato slices. This makes sure they all cook at the same rate.
Let it Rest: I know it’s tempting to dig right in. But letting it sit for 15 minutes after baking is the key to neat, perfect slices.
Cheese Choice: Gruyère is my favorite for its nutty flavor, but a good sharp white cheddar works wonderfully too.

Common Mistakes to Avoid

I’ve made these so you don’t have to! Here’s how to sidestep common pitfalls.

Rushing the Veggie Cook: Don’t crank the heat to cook the leeks and onions faster. Slow and low caramelizes their sugars and builds a sweet foundation. High heat will burn them.
Skipping the Flour Cook: When you add the flour to the butter and veggies, cook it for the full two minutes. This gets rid of the pasty taste and makes your sauce taste clean and rich.
Forgetting the Rest: Serving it straight from the oven means a soupy, messy plate. The resting time allows the starch from the potatoes to fully absorb the creamy sauce. It makes all the difference.

NUTRITION INFORMATION

Calories: 420kcal
Carbohydrates: 38g
Protein: 13g
Fat: 25g
Saturated Fat: 15g
Cholesterol: 80mg
Sodium: 480mg
Fiber: 4g
Sugar: 8g

FREQUENTLY ASKED QUESTIONS

Can I make this dish ahead of time?

Absolutely! You can assemble the entire dish, cover it, and refrigerate it for up to 24 hours before baking. You may need to add 5-10 extra minutes to the baking time since it will be cold from the fridge.

What’s the best potato to use?

I swear by Yukon Golds. They have a naturally buttery flavor and a waxier texture that holds its shape beautifully. Russets will work, but they can get a bit more mealy.

My sauce seems too thick before baking. Is that okay?

Yes, that’s perfect! The potatoes will release their starches and water as they bake, thinning the sauce out just right. A thick starting sauce is what you want.