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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. This little bonus makes it perfect for holiday entertaining!
If you are looking for a variety of side dishes for your holiday table, take a look at our Christmas recipe collection.
Creamy Potato Mash
Who doesn’t love a side of mashed potatoes to compliment their roast during the holidays?!! Creamy, buttery and all around delicious. There is only one problem. Best mashed potatoes need to be made right before serving to get the light texture right. And there isn’t always time.
To make your holiday cooking stress free and more enjoyable I bring to you my best mashed potato casserole. Make it a day head and simply reheat before serving.
Before we get into how to make it, it’s worth noting that mashed potatoes are still vital to this dish, and we want them to be as fluffy and flavourful as possible before we bring the casserole together.
The same principles that make classic mashed potatoes so enjoyable are true when we add them to a casserole. And that is BUTTER and SALT. Without these two simple yet vital ingredients, mashed potatoes will always feel underwhelming.
Because we are baking the mashed potatoes, they need to be even looser than mashed potatoes served on their own. Moisture will be pulled out when the casserole is baked, so it’s important to start with extra.
The mashed potato mixture will feel quite loose, but that’s by design. I’ve written the recipe with plenty of milk, butter and sour cream to make it right for baking.
Loaded Mashed Potatoes
As great as mashed potatoes are on their own, I couldn’t resist putting a tiny twist on the standard recipe, which reminded me of the food I grew up eating.
Potatoes and mushrooms are a classic combination in Eastern European cuisine. Inspired by that timeless pairing I added a middle layer of sautéed mushrooms and caramelised onions into my mashed spuds. It’s a combination I’ve used when frying potatoes with mushrooms and onions as well.
The idea is to find ingredients that will match with, and bring something unique to, the rich and creamy mashed potatoes. I find that the sweetness of the onions and savoury flavour of mushrooms does that perfectly.
Recipe Tips and Notes
- The main thing I learned about baked mashed potatoes is they stiffen and tend to get dry. To avoid that I added a fair amount of milk, sour cream and butter into the mash. The mashed potato mixture needs to be quite loose before it’s baked, more so than if you served them without baking.
- Use starchy or also called floury potatoes for your mash. Yukon Gold potatoes or Russet potatoes in the US fall into that category. Maris Piper and King Edward are the best ones if you make a mash in the UK. They are the most suitable to create the creamy, fluffy texture.
- To get the most out of your onions and to get that caramelised taste prepare to spend good 20 minutes sautéing them over low heat. Yes, it does take this long. In fact, it takes longer but I have a clever hack, which speeds up the process.
- Cover the onions with a lid for about 10 minutes. Once they get soft, take the lid off and get the caramelisation going!
- Cook your mushrooms and onions in butter with a good pinch of thyme. I used fresh but dried thyme will also work. And don’t forget to season well with salt and black pepper.
- Finally, to get a delicious crusty top create swirls and grooves in your mashed potatoes for added texture. Then pour melted butter all over the surface before baking.
- If the top is not golden at the end of baking time, do NOT bake it longer. This will make your mashed potatoes dry. Simply switch your oven to broil/grill mode and move the casserole closer to the element. Then broil for 1-2 minutes. Watch carefully to avoid burning.
Serving suggestions
It’s easy to argue that potato dishes like this could be served at almost any meal throughout the year. And who would complain about that?
But since it does involve extra effort to prepare and has such remarkable flavour, it’s most at home for special dinners like Thanksgiving or Christmas.
Because the flavours of this mashed potato recipe avoid any extreme, it will a great side dish choice for almost any main dish. Turkey, prime rib, ham, even duck or salmon. It’ll all work!
Storage and leftovers
If you plan on making this mashed potato casserole ahead and want to serve it the following day. Here is what you need to do. First of all, don’t fry the sage leaves until the day of serving.
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. (I used a fairly shallow casserole dish, so if yours is deeper, add additional 10 minutes to the reheating time.)
Leftovers can be kept for up to 5 days. Use and This is an affiliate link.airtight container and store in the fridge. Reheat as above, using a baking dish and an oven. Or use a microwave for simplicity, although it will compromise the golden top.
Favourite potato recipes
- Rosemary Potatoes au Gratin
- Roasted Baby Potatoes
- Garlic Mashed Potatoes
- Savoury Sweet Potato Casserole
Mashed Potato Casserole with Mushrooms and Caramelised Onions
Ingredients
- 1½ kg/3 lbs floury/starchy potatoes
- 125g/ 1/2 cup butter
- 250ml/ 1 cup whole milk or 2%
- 2½ 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 1/2 tsp dried thyme
- 4-5 sage leaves
Instructions
- 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.
Notes
Nutrition
Nutrition information is automatically calculated, so should only be used as an approximation.
Oh my goodness this looks so delicious and tasty! I can’t wait to give this recipe a try! So excited!
Enjoy, Beth!
This is so good!! Always a hit at my house!
Love hearing this!
My kids love mashed potatoes so much so I was excited to make this. They absolutely loved it! Thank you so much!
My pleasure, Lauren!!