Ingredients

1 small yellow onion (finely diced)

3 tablespoons salted butter

4 cloves fresh garlic (minced)

2 teaspoons fresh thyme leaves

2 cups heavy cream

½ cup milk

2 cups shredded gruyere cheese

½ cup grated Parmesan cheese

¼ teaspoon salt

¼ teaspoon pepper

2 ½ lbs. russet potatoes

Chives (to garnish)

Instructions

Step 1: Preheat the oven to 350°F. You can also do this later after caramelizing the onions if preferred.

Step 2: Combine the heavy cream and milk in a medium measuring cup with a spout. Warm in the microwave for 1 minute to temper, then set aside.

Step 3: In a soup pot over medium heat, add the diced onions and butter. Cook for 10 minutes, stirring often to soften the onions.

Step 4: Reduce heat to medium-low and continue cooking for another 10–15 minutes, stirring frequently, until onions brown and caramelize lightly. Add the minced garlic and thyme, cooking for 2 more minutes.

Step 5: Turn off the heat and allow the pot to cool for 1 minute. Gradually stir in the warmed heavy cream and milk mixture. Return heat to medium-high to bring the liquid to a boil, then remove from heat.

Step 6: Slowly stir in the shredded gruyere and grated Parmesan cheese, adding a little at a time to melt smoothly. Season with salt and pepper.

Step 7: Slice the potatoes into 1/8-inch thick rounds—thin slices ensure they cook through evenly. A mandoline slicer is helpful here.

Step 8: Lightly grease a 2.5-quart or 9 x 13-inch baking dish. Arrange one third of the potato slices in a single layer on the bottom. Lightly sprinkle with salt.

Step 9: Pour one third of the cheese sauce evenly over the potatoes. Repeat layers twice more: potatoes, then sauce.

Step 10: Cover the dish and bake for 1 hour. Then uncover and bake an additional 30 minutes.

Step 11: Optional: For a golden top, broil at 425°F for 1–2 minutes, watching carefully to avoid burning.

Step 12: Remove from the oven and let it rest for 5–10 minutes. The sauce will thicken as it cools. Garnish with chopped chives and serve.

Tips & Variations

Use a mandoline slicer for evenly thin potato slices, which helps them cook perfectly.

Substitute gruyere with sharp cheddar or fontina if preferred for a different flavor profile.

Add a pinch of nutmeg to the cream sauce for a warm, subtle spice note.

For a lighter version, use half-and-half instead of heavy cream and milk.

FAQs

Can I prepare au gratin potatoes ahead of time?

Yes, you can assemble the dish up to the point of baking, cover it tightly, and refrigerate for up to 24 hours before baking. Add a few extra minutes to the baking time if baking straight from the fridge.

What potatoes are best for au gratin?

Russet potatoes work best due to their starchy texture, which helps absorb the creamy sauce and results in tender, fluffy layers.