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Potato and Mushroom Gratin

Potato and Mushroom Gratin

Thinly sliced potatoes are layered with browned mushrooms, thyme, cream, and nutty gruyère for a rich, golden gratin. This comforting baked dish works beautifully as a vegetarian dinner with a crisp salad or as a special side for a holiday table.

Prep: 25 min · Cook: 75 min · Total: 100 min · Serves: 8

Ingredients

  • 0.75 tsp black pepper (freshly ground)
  • 0.125 tsp ground nutmeg
  • 1 tbsp olive oil
  • 3 cloves garlic (minced)
  • 1 oz parmesan cheese (finely grated)
  • 1 cup whole milk
  • 6 oz gruyère cheese (shredded)
  • 1 medium yellow onion (thinly sliced)
  • 2 cups heavy cream
  • 3 tbsp unsalted butter (divided)
  • 1.5 tsp kosher salt (divided)
  • 1 lb cremini mushrooms (trimmed and sliced)
  • 2 tsp fresh thyme leaves (chopped)
  • 3 lb yukon gold potatoes (peeled and sliced 1/8 inch thick)

Instructions

  1. Place a rack in the center of the oven, heat the oven to 375°F (190°C), and coat a 9-by-13-inch baking dish with 1 tbsp unsalted butter until the bottom and sides are evenly greased.
  2. Slice the 3 lb Yukon Gold potatoes 1/8 inch thick with a mandoline or sharp chef's knife, then keep the slices in a large bowl of cold water for up to 10 minutes while you prepare the mushrooms so they do not discolor.
  3. Heat 1 tbsp olive oil and 1 tbsp unsalted butter in a 12-inch skillet over medium-high heat until the butter foams, about 1 minute.
  4. Add the 1 lb sliced cremini mushrooms to the skillet in an even layer and cook, stirring every 2 minutes, until their liquid evaporates and the mushrooms are browned at the edges, 8-10 minutes.
  5. Reduce the skillet heat to medium, add the thinly sliced yellow onion, and cook with the mushrooms, stirring often, until the onion is softened and lightly golden, 5-6 minutes.
  6. Add the 3 minced garlic cloves, 2 tsp chopped fresh thyme leaves, 0.5 tsp kosher salt, and 0.25 tsp black pepper to the skillet, then cook over medium heat for 1 minute until fragrant; remove the skillet from the heat.
  7. Drain the potatoes in a colander and pat them very dry with a clean kitchen towel, because excess water will thin the cream sauce and prevent the gratin from setting.
  8. Combine the 2 cups heavy cream, 1 cup whole milk, 0.125 tsp ground nutmeg, remaining 1 tsp kosher salt, and remaining 0.5 tsp black pepper in a medium saucepan, then warm over medium heat for 4-5 minutes until steaming but not boiling.
  9. Arrange half of the potato slices in the prepared 9-by-13-inch baking dish in slightly overlapping rows, then spread all of the mushroom-onion mixture evenly over the potatoes with a spatula.
  10. Layer the remaining potato slices over the mushrooms, pour the hot cream mixture evenly over the top, and press gently with a spatula so the potatoes are mostly submerged.
  11. Cover the baking dish tightly with aluminum foil, place it on a rimmed sheet pan, and bake at 375°F (190°C) for 40 minutes until the potatoes are beginning to soften when pierced through the foil with a paring knife.
  12. Remove the foil, sprinkle the 6 oz shredded Gruyère cheese and 1 oz grated Parmesan cheese evenly over the gratin, and continue baking uncovered for 25-30 minutes until the top is deep golden brown, the cream is bubbling around the edges, and a paring knife slides easily through the center potatoes.
  13. Let the gratin rest on a wire rack for 10-15 minutes before serving so the cream can thicken and the slices hold together cleanly.

Nutrition (per serving)

  • Calories: 493
  • Protein: 13 g
  • Fat: 35 g
  • Carbohydrates: 34 g
  • Fiber: 4 g
  • Sugar: 6 g
  • Sodium: 620 mg

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