Cheesy Potato Gratin (Scalloped Potatoes)

You'll need:

• 8 tablespoons (1 stick) unsalted butter
• 1/2 yellow onion, finely diced
• 1 tsp minced garlic
• 1/2 cup flour
• 1 quart milk
• 1/4 cup Dijon mustard
• 2 cups grated Gruyere
• 1 cup grated Parmesan (get the refrigerated stuff, not the powder in a bottle!)
• 4 large russet potatoes, sliced to 1/4 inch
• Salt to taste

Prepare the ingredients: Preheat the oven to 375 degrees F. Line a 9x13 pan with aluminum foil and spray with cooking spray.

Dice the onion - I personally like to grate it on the large side of a box grater, so that the onion melts into the sauce instead of keeping any crunch. You can dice it if you want pieces of onions in the sauce, though. Mince the garlic and set aside in a separate small bowl.

Grate the gruyere and parmesan.

Warm the milk in a microwave or a small saucepan on the stove (do not boil).

Do not slice the potatoes yet! Wait until the bechamel is done.

Make the bechamel: (Click here for a detailed tutorial on a basic bechamel.) Put a large saucepan over medium heat and melt the butter. As soon as it's melted, add the onions and cook, stirring occasionally, until the onions are soft, about 5 minutes. Add the garlic and stir until fragrant - another minute or two.

Whisk in the flour until smooth and cook 2 to 3 minutes - don't cook it too much or the sauce won't thicken. Then pour in the warm milk and cook on medium until it just starts to bubble, about 5 minutes.

Remove from heat and stir in mustard and cheeses. Season with salt - I added 2 to 3 tsp - tasting as you go.

Prepare the gratin: Wait until the sauce is done to slice the potatoes, as they will brown if you leave them out. Wash the potatoes - no need to peel them. Slice them to about 1/4 inch thick - you can do this with a knife instead of a mandolin, since you don't want them super super thin.

As you finish slicing a potato, go on and layer the slices on the bottom of the 9x13 pan. I use about one potato per layer. Once you have a flat layer of potato slices, ladle the cheese sauce over it and make sure all the potatoes are covered. Continue until you use all the potato slices, topping with a thick layer of bechamel.

Bake until potatoes are tender - about an hour. Let sit for about 20 minutes (or more) before serving - the sauce will thicken as it sits. Serve!

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Cheesy Potato Gratin

Author: Mallory

Prep time: 15 MinCook time: 60 MinInactive time: 20 MinTotal time: 1 H & 34 M

Ingredients

• 8 tablespoons (1 stick) unsalted butter
• 1/2 yellow onion, finely diced
• 1 tsp minced garlic
• 1/2 cup flour
• 1 quart milk
• 1/4 cup Dijon mustard
• 2 cups grated Gruyere
• 1 cup grated Parmesan 
• 4 large russet potatoes, sliced to 1/4 inch
• Salt to taste

Instructions

Prep the ingredients:

1. Preheat the oven to 375 degrees F. Line a 9x13 pan with aluminum foil and spray with cooking spray.
2. Dice the onion - I personally like to grate it on the large side of a box grater, so that the onion melts into the sauce instead of keeping any crunch. You can dice it if you want pieces of onions in the sauce, though. Mince the garlic and set aside in a separate small bowl.
3. Grate the gruyere and parmesan.
4. Warm the milk in a microwave or a small saucepan on the stove (do not boil).

Make the bechamel:

1. Put a large saucepan over medium heat and melt the butter. As soon as it's melted, add the onions and cook, stirring occasionally, until the onions are soft, about 5 minutes. Add the garlic and stir until fragrant - another minute or two.
2. Whisk in the flour until smooth and cook 2 to 3 minutes - don't cook it too much or the sauce won't thicken. Then pour in the warm milk and cook on medium until it just starts to bubble, about 5 minutes.
3. Remove from heat and stir in mustard and cheeses. Season with salt - I added 2 to 3 tsp - tasting as you go.

Prepare the gratin:

1. Wait until the sauce is done to slice the potatoes, as they will brown if you leave them out. Wash the potatoes - no need to peel them. Slice them to about 1/4 inch thick - you can do this with a knife instead of a mandolin, since you don't want them super super thin.
2. As you finish slicing a potato, go on and layer the slices on the bottom of the 9x13 pan. I use about one potato per layer. Once you have a flat layer of potato slices, ladle the cheese sauce over it and make sure all the potatoes are covered. Continue until you use all the potato slices, topping with a thick layer of bechamel.
3. Bake until potatoes are tender - about an hour. Let sit for about 20 minutes (or more) before serving - the sauce will thicken as it sits. Serve!

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