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Mashed Potato Casserole with Mushrooms showing in the middle layer

Mashed Potato Casserole with Mushrooms and Caramelised Onions

This creamy mashed potato casserole loaded with caramelised onions, mushrooms and topped with fried sage leaves is a brilliant dish, that could be made ahead of time.
5 from 3 votes
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Course: Side Dish
Cuisine: American, Russian
Keyword: mashed potato casserole
Prep Time: 20 minutes
Cook Time: 50 minutes
Servings: 10
Calories: 337kcal


  • 1.5 kg/3 lbs floury/starchy potatoes
  • 125g/ ½ cup butter
  • 250ml/ 1 cup whole milk or 2%
  • 2.5 tsp salt
  • 125ml/1/2 cup sour cream
  • 2 eggs beaten
  • 6 tbsp butter divided
  • 1 onion sliced
  • 1-2 cloves garlic
  • 600g/5 cups button mushrooms sliced
  • 4 sprigs fresh thyme or ½ tsp dried thyme
  • 4-5 sage leaves


  • Peel and cut potatoes, boil in salted water until fork tender. Drain and air steam your potatoes until they look dry and flaky. Mash with hot milk, butter, sour cream and salt, then fold in beaten eggs. The mixture will look runny but that's exactly what you are looking for.
  • While the potatoes are boiling. Saute the onions in 2 tablespoons of butter over low heat, covered with a lid for 10 minutes until tender, then uncover and add the mushrooms with thyme and garlic, stirl and continue cooking for 10 minutes longer until all liquid evaporates and the onions and mushrooms are caramelised.
  • In a 9X13 inch or 3 Quart/Litre casserole dish layer the mashed potatoes with the mushroom and onion layer in the middle. Top the 2 tbsp of melted butter. Bake uncovered at 180C/350F for 25 minutes until the casserole is bubbling and puffed up. Broil for 1-2 minutes for a crispy top.
  • Before serving fry the sage leaves in the remaining 2 tablespoons of butter. Watch the sage leaves carefully as they burn quickly. Remove from the butter as soon as they start to curl and top the casserole. Reserve the sage flavoured butter for another use.


Reheating Instructions if you make this casserole one day ahead:
Cover your casserole with aluminum foil and reheat in the preheated to 180C/350F oven to 25 minutes, then take the foil off and heat for additional 5-7 minutes.


Calories: 337kcal | Carbohydrates: 31g | Protein: 7g | Fat: 22g | Saturated Fat: 13g | Cholesterol: 87mg | Sodium: 661mg | Potassium: 901mg | Fiber: 4g | Sugar: 4g | Vitamin A: 710IU | Vitamin C: 32mg | Calcium: 76mg | Iron: 2mg