This Potato and Celery Root Gratin is my take on a baked gratin that’s taken to the next level with a combination of thinly sliced potatoes and celery root, baked in a creamy, cheese sauce. This is a truly delicious potato side dish that’s perfect for a family or holiday dinner.
Truth be told, I could honestly eat this as a meal. I’m sure many of you will agree that there’s nothing wrong with creamy, cheesy potato comfort food. Fall and winter is the perfect season for this dish.
What is celery root/celeriac?
It is the root of the celery that is hard like a carrot or parsnip and tastes just like celery.
Celery root (or celeriac) is easy to find. You may have seen it in the produce area of your supermarket and wondered what that ugly thing is (pic below). Trust me, celery root is a delicious combination with the potato; it just adds a nice, mild celery flavor.
This is a French style au gratin that is similar to scalloped potatoes. Both are baked in a milk/cream sauce, but the gratin always has some kind of cheese added.
Gratin is not limited to being made with potatoes. One of my favorite fall and winter side dishes is my Butternut Squash and Sage Gratin that has a delicious crispy panko breadcrumb topping.
My method is a little different from the traditional gratin method. I thicken the milk and add cheese before baking (basically making a cheese sauce). I find that this method avoids the curdling look that can happen from the high heat of the oven.
What potatoes are best for gratin?
All potatoes! Russet, gold and even sweet potatoes will work.
How to cut thin potatoes for gratin
A mandolin on the thinnest setting is the best way. A mandoline is a great chef slicing tool for slicing anything to uniform thickness.
How long does it take to bake a gratin?
This depends on how thick your potato slices are. But, plan on at least 1 hour. This is why it is best to cut them as thinly as you can.
Sliced potatoes and celery root are baked in a creamy cheese sauce with garlic and rosemary.
Prep Time15 minutes
Cook Time1 hour15 minutes
Total Time1 hour30 minutes
3 tablespoons butter, plus more for greasing
3 tablespoons all-purpose/plain flour
1 garlic clove, peeled and finely chopped
1 ½ teaspoons fresh rosemary, chopped
1 ½ cups (354 ml) whole milk, room temperature
½ teaspoon salt
1 ½ pounds (680 grams) potatoes, peeled and thinly sliced *see note
½ pound (226 grams) celery root/celeriac, peeled and thinly sliced
1 cup/3 ounces (86 grams) white cheddar cheese, grated
½ cup/1 ½ ounces (45 grams) Parmesan cheese, grated
Preheat oven to 375°F/190°C. Generously butter an 11-inch (28 cm/1 ½ quart ) oval dish.
Melt the 3 tablespoons butter in a pan over medium heat. Whisk in the flour until it absorbs the butter. Cook while stirring for 2 minutes to cook out the flour taste, it will start to turn darker when ready. Stir in the garlic and rosemary, don’t burn the flour or the garlic. Slowly whisk in the milk until thickened and there are no lumps. Add half the cheddar cheese and stir until completely melted, add the rest of the cheddar and stir until sauce is smooth. Taste the sauce and add salt if needed. This will depend on how salty your cheese is. Keep warm over very low heat.
Add the half the potato slices to the baking dish, slightly overlapping in 2 layers, then top with the all of the celeriac slices, slightly overlapping. Pour over half of the sauce. Add the rest of the potato slices and pour over the rest of the sauce pushing the sauce into all the nooks and crannies. Sprinkle the top evenly with the Parmesan cheese.
Cover tightly with foil and bake for 1 hour, or until the potatoes are tender when pierced with a sharp knife. Remove foil and continue to bake for 15 more minutes until golden brown and bubbly. Time will depend on how thin the vegetables are sliced. If you would like it more browned, you can broil for a couple of minutes (or oven grill if you’re in the U.K).
Note: A mandoline on the thinnest setting is best for this.