Potato and Mushroom Gratin

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Sliced mushrooms, gruyere cheese and potatoes in a basket are being prepped for Potato and Mushroom Gratin.

With Easter just around the corner, you’ll want to be dinner ready for family and friends. Hopefully, with some luck, the Covid-19 gathering restrictions will ease in my area and a modest gathering can take place. I do miss my family and friends, and I miss cooking for them. It’s the best part of any gathering, in my opinion. You’re gonna love this easy and delicious Potato and Mushroom Gratin. It feeds 8 people, it’s easy to assemble and bakes up golden and creamy every time. There’s no messing around with cooking a white sauce, there’s no boilovers and no pre-cooking potatoes. It’s a no-fuss casserole, loaded with earthy mushrooms and Gruyere cheese, and has become my favourite to serve with ham. It’s also fantastic served with roast chicken, pork or grilled steak.

Sauteed mushroom in a stainless steel skillet that have been cooked for Potato and Mushroom Gratin.
A prepped and layered Potato and Mushroom Gratin in a red casserole dish ready for the oven.

What makes this potato casserole a winner in my books is the simplicity of the sauce. The mixture of cream and sour cream makes it extra creamy and the sour cream gives a little tang. The sauce is whisked together cold and poured over the potato slices. I find there’s no need to be fussy with the potato arrangement either. Simply toss the slices into the casserole dish and spread them around evenly instead of trying to layer them ever so carefully. If you don’t have a mandolin for thinly slicing the potatoes, that’s no problem, simply slice as thin as you can by hand and cook for the longest time suggested in the recipe. Remember, dinner doesn’t need to be fussy. This simple casserole is not only delicious, but it also feeds a small crowd and is worthy of your family gathering. Enjoy!

Potato and Mushroom Gratin baked golden in a red casserole dish and garnished with chopped green onions.

Author:  Compelled to Cook

 

Potato and Mushroom Gratin baked golden in a red casserole dish and garnished with chopped green onions.
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Potato and Mushroom Gratin

Prep Time 30 minutes
Cook Time 1 hour 10 minutes
Resting Time 10 minutes
Total Time 1 hour 50 minutes
Servings 8 people

Ingredients

  • 3-4 tbsp unsalted butter plus more for dish
  • 16 ounces mushrooms, thinly sliced see note
  • 1 small onion, thinly slice
  • 3 cloves garlic, minced
  • 1 cup heavy cream
  • 1 cup sour cream, full fat
  • 1 tsp kosher salt
  • 1/2 tsp pepper
  • pinch nutmeg
  • 1/3 cup milk
  • 2 tbsp flour
  • 2 1/2 pounds russet potatoes see note
  • 6 ounces Gruyere cheese, shredded

Instructions

  • Preheat oven to 375°F and butter a large 3-quart casserole dish.
  • Melt 2 tbsp butter in a large skillet over medium-high heat until starting to foam. Add mushroom and cook until lightly browned and the moisture has evaporated. Add minced garlic and continue to cook, stirring often for 1-2 minutes. Season with salt and pepper. Remove to a clean bowl.
  • Melt another 1-2 tbsp of butter over medium heat and add sliced onions. Sprinkle with salt and sauté for 4-5 minutes until starting to soften.
  • While mushrooms and onions are cooking, whisk together cream, sour cream, salt, pepper and nutmeg. In a separate bowl whisk together milk and flour until smooth. Add to the sour cream mixture and whisk until incorporated. Grate the cheese and set it aside.
  • Peel and thinly slice the potatoes and dab dry with a paper towel. Spread half of the potato slices evenly in the bottom of the prepared casserole dish. Scatter half of the sauteed mushrooms and onions over the potatoes. Top evenly with half of the cheese. Repeat with remaining potatoes, mushrooms and cheese.
  • Pour the cream mixture evenly over the potatoes. Give the pan a gentle shake to help flatten everything out. Cover and bake for 45-60 minutes (this will depend on how thin you've sliced the potatoes) Start checking at 45 minutes. The potatoes should be "just" tender in the center. Remove the cover and bake for an additional 20 minutes until the top is golden and the potatoes are cooked through.
  • Remove from oven, cover and allow to rest for 10 minutes before serving. Garnish with chopped fresh parsley or sliced green onions if desired.

Notes

-The cooking of the mushrooms and onions, along with the mixing of the sauce can be done several hours and up to one day in advance. So all you have left to do is slice the potatoes, assemble and bake the casserole.
-Russet potatoes work well for this recipe, however, I've also used red potatoes with equally good results.
-The covered cooking time will vary slightly depending on how thin you've sliced the potatoes.  I find cutting them by hand instead of using a mandolin requires closer to 60 minutes covered.
-I like using a mix of cremini and white mushrooms for this recipe but either/or will work well.

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