Cozy and Comforting Potato and Leek Gratin

A cozy side dish built on simple, rich layers

Potato and leek gratin is the kind of side dish that feels both familiar and special: tender slices of potato, softened leeks, and a sauce that turns creamy and cohesive in the oven. In this version, a straightforward cheese sauce coats the vegetables from the start, helping the finished gratin bake up evenly and taste consistently rich from edge to center. The combination of Gruyère, leeks, and potatoes forms the foundation, while a few supporting ingredients bring balance and warmth.

What makes this gratin particularly comforting is the way the sauce “cocoons” the potatoes and leeks. Rather than relying on dry layering and hoping the liquid finds its way through, the sliced potatoes are added directly into the cream mixture as you cut them. That step helps ensure each slice is coated on both sides and prevents clumps of stuck-together potatoes that can lead to uneven cooking.

Flavor profile: cheesy, aromatic, and balanced

This dish is designed to be cheesy and punchy without becoming overwhelming. Gruyère provides the core flavor and melts smoothly into the sauce. Fresh thyme adds an aromatic note that reads as cozy and savory, and a little extra thyme can be used as garnish at the end. Dijon mustard and garlic help cut through the richness, while nutmeg adds a subtle warmth. The seasonings are rounded out with pepper and paprika, creating a sauce that tastes layered rather than simply “creamy.”

The leeks contribute sweetness and depth, especially when cooked until softened and just starting to brown at the edges. That gentle browning adds complexity before the gratin even goes into the oven.

Key ingredients

• Unsalted butter (plus more for the baking dish)
• Leeks (using the white and light green parts; dark green parts are saved for another use)
• Garlic, finely chopped
• Fresh thyme (plus extra leaves for garnish)
• Cream
• Dijon mustard
• Pepper
• Nutmeg
• Paprika
• Salt
• Gruyère cheese, shredded
• Potatoes, peeled and thinly sliced

Prep notes that make a difference

Leeks can hold grit between their layers, so rinsing matters. After slicing the leeks in half lengthwise, give them a good rinse under running water to help avoid any grit in the finished gratin. Once cleaned and sliced, the leeks are cooked in butter with salt until softened and beginning to brown at the edges, which takes about 10 to 12 minutes. Garlic and thyme are added near the end so they become fragrant without overcooking.

For the potatoes, thin slicing is essential. The slices are cut to about 1/16- to 1/8-inch thick, which supports even tenderness by the time the top turns golden. As you slice, add the potatoes directly to the cream mixture and gently stir. The goal is simple but important: make sure no slices are stuck together and that every piece is coated.

Step-by-step method overview

• Separate the white and light green parts of the leeks from the dark green parts (reserve the dark green parts for another use). Slice the usable portions and rinse well to remove grit.
• Melt butter in a large deep-sided skillet over medium heat. Add leeks and salt, then cook until softened and starting to brown at the edges (about 10 to 12 minutes).
• Add garlic and chopped thyme and cook until fragrant (about 1 minute).
• Stir in cream, mustard, pepper, nutmeg, paprika, and remaining salt. Bring to a simmer and let it simmer for 1 minute.
• Remove from heat and stir in Gruyère until melted, forming a smooth cheese sauce.
• Preheat the oven to 350°F and butter a 2- to 3-quart baking dish.
• Peel and thinly slice the potatoes. Add slices to the cream mixture as you cut them, stirring gently so each slice is coated and not stuck together.
• Transfer the mixture to the prepared baking dish, then shake the dish to help the potatoes settle into an even layer.
• Cover tightly with aluminum foil and bake until the potatoes are starting to become tender (45 to 50 minutes).
• Uncover and continue baking until the potatoes are tender and the top is golden (25 to 35 minutes).
• Let the gratin rest for 10 minutes, then garnish with thyme leaves.

Timing, texture, and resting

The baking is done in two stages: covered first to help the potatoes soften, then uncovered to finish cooking and develop a golden top. Resting for 10 minutes after baking is part of the process, not an afterthought. It gives the gratin time to settle so the layers hold together better when served, while still staying creamy.

Make-ahead and storage

This gratin is practical for entertaining or meal planning because it can be assembled in advance. You can prepare it (without baking) up to 1 day ahead and store it tightly covered in the refrigerator. If you bake it straight from cold, plan for an additional 10 to 15 minutes of baking time.

Leftovers keep well. Store them in an airtight container in the refrigerator for up to 3 days.

Serving idea

As a side dish, this potato and leek gratin delivers creamy layers, savory aromatics, and a golden finish. With its Gruyère-forward sauce and thyme garnish, it’s designed to feel cozy and comforting while still tasting balanced and deliberate.