This cheesy mashed potato au gratin is pure indulgence – the perfect potato side dish for a big occasion such as Thanksgiving or Christmas dinner, and quite possibly the best way to enjoy potatoes ever. It features a base of fluffy mashed potatoes topped with thinly sliced potatoes and cheese, then baked until golden and crispy. Delicious!

It’s perfect for big dinners – Thanksgiving, Christmas, or just a cozy weekend meal. You get the fluffiness of mashed potatoes underneath, and that golden, crisp layer on top that makes you want to break through with a spoon right away.

If you’re looking for even more ideas for big dinners, I’ve included this and other side dishes in my 15 Delicious Thanksgiving Side Dish Recipes to Try roundup. You might also love my popular potato and leek gratin dish, or pair this recipe with my honey butter carrots or these simple roasted green beans.

Ingredient Notes

The exact quantities of the ingredients you need for this recipe are included in the recipe card at the bottom of this post. Below are a few notes on the ingredients and potential substitutions:

• Baking Potatoes: Baking potatoes like Russets are best because of their starchy texture, which creates fluffier mashed potatoes. Yukon Golds or any other white potato can be substituted for a creamier texture.
  
• Butter: Adds richness and flavor to both the mashed potatoes and the crispy topping! A must.
  
• Garlic: Freshly minced garlic gives the garlic butter a bold flavor, which I feel like you need with the amount of potatoes we’re working with in this dish, but garlic powder could work in a pinch if you’re out of fresh garlic.
  
• Cheddar Cheese: A sharp cheddar works best for its melty, tangy flavor. You can swap it for a milder cheddar or even mozzarella for a more subtle flavor.
  
• Pecorino Cheese: Parmesan cheese can be used as a substitute if Pecorino isn’t available, but be aware that Parmesan often isn’t vegetarian-friendly!
  
• Crème Fraîche: Adds creaminess and tang to the mashed potatoes. Sour cream is a great substitute if needed.

Variations & Substitutions

This mashed potato au gratin is versatile and easy to tweak depending on your preferences:

• To make a vegan version, use plant-based butter, dairy-free cheese, and a cashew cream or vegan sour cream in place of crème fraîche.
  
• Add herbs for more flavor – fresh thyme, rosemary, or chives work well mixed into the mash or sprinkled on top.
  
• Feel free to swap out the cheeses – Gruyère, fontina, or a sharp white cheddar are great alternatives.

How To Make This Mashed Potato Au Gratin

The exact step-by-step instructions and ingredient quantities will be in the recipe card at the bottom of this page. Below is an overview of how to make this dish, plus some helpful tips.

Step 1) Boil the Potatoes

Peel and chop 4 of the potatoes into chunks (reserve the other potato for slicing thinly on top). Add them to a saucepan with cold water and salt, then boil for 10-15 minutes until fork tender.

Step 2) Prepare the Topping

While the potatoes are boiling, slice the remaining potato into thin rounds. This will create a crispy layer on top of the mashed potatoes – taking your baked mashed potatoes from good to outstanding!

Step 3) Make Garlic Butter

Soften the butter in the microwave (just soften, not melt) and mix it with minced garlic and dried parsley.

Step 4) Mash the Potatoes

Drain the boiled potatoes, then mash with three-quarters of the garlic butter, half the cheddar cheese, and the crème fraîche until smooth and fluffy. Season with salt and pepper to taste.

Step 5) Assemble the Dish

Grease your oven-proof dish with a little garlic butter, then add the mashed potatoes, pressing down gently to create an even layer.

Step 6) Add the Topping

Layer the potato slices on top of the mashed potatoes, slightly overlapping them like you would in a hotpot or traditional au gratin. Brush the top with the remaining garlic butter and sprinkle over the remaining cheddar and Pecorino cheese.

Step 7) Bake

Cover the dish with foil and bake at 200°C (395°F) for 45 minutes, then remove the foil and bake uncovered for another 25 minutes until the topping is golden brown and crispy.

Top Tips \ Chef’s Notes

• Slicing the potatoes: If you want perfectly even slices, a mandolin is ideal. However, this is still one of those gadgets I’ve never actually gotten around to adding to my kitchen drawer, so I just use a chopping board and knife to slice my potatoes thinly. Be careful and use a fork to hold the potato in place when it gets too small to be safe to hold it with your fingers.
  
• Softened butter, not melted: Softening the butter instead of melting it ensures it coats the potatoes and garlic evenly without becoming too runny.
  
• Let the dish rest for a few minutes before serving so the layers hold together nicely.
  
• Season to taste: Potatoes absorb a lot of salt, so be sure to taste the mashed potatoes before assembling the dish and adjust the seasoning as needed.
  
• 2-3 minutes under the broiler can deepen the top color – but watch it carefully so it doesn’t burn.

Storage

This mashed potato au gratin is perfect for making ahead of time. I often assemble it earlier in the day, or the day before, cover it, and pop it in the fridge until I’m ready to bake. It makes dinner prep so much easier, especially if you’re prepping multiple other side dishes!

When I’m reheating, I like to use the oven at 180°C (350°F) because it keeps that lovely crisp topping intact – cover with foil if you’re afraid the top will brown. The microwave works too if you’re short on time, though the top will soften a bit.

How To Serve?

This mashed potato au gratin makes a fantastic side dish for a roast dinner or special occasion like Christmas or Thanksgiving. Serve it with your choice of main, alongside roasted green beans, honey butter carrots, and homemade stuffing balls!

FAQs

Can I make this dish ahead of time?
Yes, you can assemble the mashed potatoes and topping in the dish ahead of time. Store it in the refrigerator, covered, and bake it when you’re ready to serve.

Can I freeze mashed potato au gratin?
Yes, you can freeze this dish after baking. Let it cool completely, cover tightly, and freeze for up to 2 months. Thaw in the refrigerator overnight and reheat in the oven until warmed through.

What other cheeses can I use?
Feel free to experiment with cheeses! Gruyère, fontina, or even a sharp white cheddar would work well in this dish.

Shout out to Chris from Don’t Go Bacon My Heart, whose gorgeous version of this recipe inspired me. I believe he, in turn, was inspired by Chef John from Food Wishes.