Potato and Onion Bake in a Mornay Sauce

Move aside mashed potatoes! This creamy potato and onion bake with a mornay sauce is stealing the side dish show this holiday season.

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What Exactly Is a Potato Bake?

This dish is a unique blend of scalloped potatoes, potatoes au gratin, and dauphinoise potatoes.

• Scalloped Potatoes: A simpler version without cheese layers, using a roux of milk, cream, and flour.
• Potatoes Au Gratin: A more decadent dish featuring cheesy layers and breadcrumbs.
• Dauphinoise Potatoes: Made with uncooked, thinly sliced potatoes cooked in a cream sauce.

My potato bake combines uncooked potatoes, layered with onions and a mornay sauce, topped with a mix of cheeses. While it incorporates elements from all three dishes, it stands alone as a delicious potato and onion bake—akin to a casserole but more sophisticated.

Jump to:

• What Exactly Is a Potato Bake?
• Why You'll Love This
• Ingredients
• Substituions & Variations
• Equipment
• Instructions
• Serving Suggestions
• Storage
• FAQ
• Related
• Potato and Onion Bake in a Mornay Sauce

Why You'll Love This

Potatoes are a staple side dish for any holiday, from mashed to scalloped. This season, I’m introducing a new recipe that blends elements of scalloped potatoes, potatoes au gratin, and dauphinoise potatoes into a unique potato bake.

With a creamy mornay sauce, onions, and a cheesy finish, this dish is decadently delicious and will elevate your holiday menu. Try it yourself and see what all the hype is about!

Ingredients

Here is everything you will need to make this recipe come to life!

For the Mornay Sauce:

• Butter creates a rich, smooth base.
• Gluten-free flour thickens the sauce.
• Coconut milk adds creaminess and a dairy-free option.
• Gruyere cheese provides a nutty, melty richness.
• Parmesan cheese enhances depth with sharpness.
• Thyme adds earthy aroma.
• Black pepper balances richness with mild heat.
• Salt enhances flavor.
• Paprika gives smokiness and color.
• Nutmeg adds warmth and sweetness.

For the Potatoes:

• Russet potatoes absorb the sauce and become tender.
• White onion adds sweetness and savory depth.
• Gruyere cheese enhances creaminess and nuttiness.
• Parmesan cheese creates a golden, crispy crust.
• Butter adds richness and tenderness.
• Parsley offers a fresh, herbal finish.

See recipe card below for a full list of ingredients and measurements!

Substituions & Variations

These tips below can help you adjust the recipe!

• Dairy Free: Replace Gruyere and Parmesan with vegan cheese (like dairy-free cheddar or mozzarella) for a dairy-free option. Use almond milk or cashew cream instead of coconut milk for a more neutral, creamy base.
• Spice Alternatives: Swap thyme for rosemary for a piney herbal flavor, and replace paprika with cayenne pepper for heat or smoked paprika for a smoky depth.

Please note that this recipe has not been tested with any other substitutions or variations than the ones listed above.

Equipment

All you will need for this recipe includes; 9" by 13" baking dish, a mandolin and a large pot!

Instructions

What is a Mornay Sauce

You may know about roux and béchamel sauce, but have you heard of mornay sauce?

A roux is a mix of flour and butter, while béchamel sauce is made by adding milk (or heavy cream) and spices like nutmeg to a roux. Mornay sauce is the next step, created by adding grated cheese—typically gruyère and parmesan, though white cheddar can also be used—to béchamel.

How to Make the Mornay Sauce

Start by making the roux: melt six tablespoons of unsalted butter in a large pot over medium heat. Gradually whisk in one-fourth cup of gluten-free all-purpose flour, adding one tablespoon at a time to avoid clumps.

Next, add milk to create the béchamel sauce. I prefer using one 15 oz. can of unsweetened coconut milk. Start by whisking in half the can until smooth, then add the rest.

To transform it into a mornay sauce, mix in three-fourths cup of grated gruyère and half a cup of grated parmesan cheese, adding half at a time and whisking until melted. Fold in the cheese slowly- it will take time to incorporate all of the cheese.

Finally, season the mornay sauce with two teaspoons of dried thyme, one and a half teaspoons of black pepper, half a teaspoon of nutmeg, three-fourths teaspoon of paprika, and one teaspoon of salt. Remove from heat once combined.

Layering the Potatoes

Before we can layer the potatoes, we first need to prepare the baking dish. Take a nine by thirteen inch baking dish and grease it. Make sure you grease the edges too as the potatoes will be stacked to the very top! Then, preheat the oven to 375 degrees Fahrenheit.

Now, it's time for the potatoes! Start by washing and peeling four russet potatoes. Then use a mandolin slicer to cut the potatoes into half an inch thick slices. This not only ensures that each potato is the same thickness, but also makes the process much quicker! Once all of the potatoes are sliced, set them to the side. Julienne half of a white onion and place by the potatoes.

To layer the potatoes, start by covering the bottom of the dish with about 3-4 rows of potatoe, laid against one another to create a domino effect. Once there's a layer of potatoes, sprinkle the julienne white onion over the potatoes. There should be roughly five to ten pieces of onion spread out over the potatoes. Finally, whisk the mornay sauce and pour it over the potatoes and onions. Pour a little less than one fourth of the sauce over the layer, then use a spatula to disperse it. Repeat this process until you get to the very top of the dish and have done your final layer of potatoes, onions, and mornay sauce. To finish the dish, sprinkle a mixture of half a cup of grated parmesan cheese and half a cup of grated gruyere cheese over to the top.

The Last Steps

Before you pop the potato and onion bake in the oven, slice three tablespoons of butter into five even pieces. Place four pieces of the butter on the edges of the dish and one in the center. This will prevent the edges from burning and ensure the center of the dish is perfectly crispy. Cover the bake with foil and place it in the center of the oven. We're going to bake the potatoes for an hour and a half covered at 375 degrees. After the allotted time, remove the foil and allow the potatoes to cook for another 15 minutes uncovered. Finally, broil the potatoes on high for five minutes . Once the potatoes are golden brown, turn off the oven and remove the dish. Make sure to watch the potatoes closely as they can burn quickly on broil!

Serving Suggestions

Allow the dish to cool for at least twenty minutes. Finally, before you serve the potato and onion bake, garnish with freshly minced parsley.

Storage

Store leftovers in an airtight container in the fridge for 3-4 days, or freeze for 2-3 months; reheat in the oven or microwave.

FAQ

Related

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