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Creamy Mushroom and Spinach Potato Gratin

A rustic, comforting bake featuring thinly sliced Yukon Gold potatoes layered with sautéed mushrooms and spinach, baked in a garlic-herb cream sauce and finished with a golden Gruyère and Parmesan crust.
Print Recipe
Prep Time 30 minutes
Cook Time 1 hour 15 minutes
Total Time 2 hours 5 minutes
Servings: 8 servings
Course: Dinner, Side Dish
Cuisine: American Comfort, French Inspired
Calories: 420
Ingredients Equipment Method Notes

Ingredients
  

Vegetable Layers
  • 2 lbs Yukon Gold potatoes scrubbed and sliced 1/8-inch thick
  • 1 lb cremini mushrooms sliced
  • 5 oz fresh baby spinach
  • 2 tbsp unsalted butter
  • 1 tbsp olive oil
  • 3 cloves garlic minced
Cream Sauce & Topping
  • 1.5 cups heavy whipping cream
  • 0.5 cup whole milk
  • 2 tsp fresh thyme leaves plus extra for garnish
  • 0.25 tsp ground nutmeg
  • 1.5 tsp salt divided use
  • 0.75 tsp black pepper freshly ground, divided use
  • 6 oz Gruyère cheese freshly grated
  • 2 oz Parmesan cheese freshly grated

Equipment

  • Mandoline Slicer (optional but recommended)
  • Large Skillet
  • 2.5 to 3-quart Baking Dish
  • Cheese grater

Method
 

  1. Preheat oven to 375°F (190°C). Grease a 2.5-3 quart baking dish. Slice potatoes 1/8-inch thick (do not rinse). Mix cheeses together.
  2. In a large skillet, heat butter and oil. Sauté mushrooms until deeply browned (8-10 mins). Add garlic, cook 30 seconds. Add spinach and wilt (1-2 mins). Season with 1/2 tsp salt and 1/4 tsp pepper.
  3. Combine cream, milk, thyme, nutmeg, remaining 1 tsp salt, and remaining 1/2 tsp pepper. Warm gently.
  4. Layer half the potatoes in the dish. Top with the mushroom/spinach mix and 1/3 of the cheese.
  5. Layer remaining potatoes on top. Pour the warmed cream mixture over everything. Top with remaining cheese.
  6. Cover tightly with foil (greased on underside). Bake for 45 minutes.
  7. Remove foil. Bake uncovered another 25-35 minutes until potatoes are tender and top is golden brown.
  8. Let rest for at least 15-20 minutes before serving to allow sauce to set.

Notes

Do not rinse potato slices; their starch is needed to thicken the sauce. Sautéing mushrooms and spinach first is crucial to prevent a watery gratin. Gruyère is recommended for its melting qualities and nutty flavor.