This savory Mushroom, Caramelized Onion, and Mashed Potato Tart (or pie or galette) has a flaky all-butter pie crust that gets filled with mashed potatoes, goat cheese, golden brown mushrooms and flavorful caramelized onions. Baked and topped with fresh thyme, this rustic pie is perfect as a meal, side, or appetizer.
This potato tart with goat cheese and thyme is the perfect savory dish to use for any occasion. While called a tart, it can also most definitely be called a pie or galette. Baked in a cast iron skillet rather than a tart pan or pie dish, this recipe can also very simply be made right on a sheet pan.
Rustic and delicious, a flaky all-butter pie crust is filled with mashed potatoes, then sprinkled with goat cheese (or gorgonzola or parmesan), browned mushrooms, and caramelized onions. Top the whole thing off with fresh thyme and bake until the crust is golden brown and perfect.
Upon slicing in, you see the distinct layers of each. The flavors and textures merry perfectly, leaving you going back for one more bite time and time again! It's crunchy, creamy, sharp and sweet all at the same time. Delicious!
- Pie crust. This recipe uses my all-butter pie crust. I highly suggest making this, as it provides the best flavor and texture.
- Mashed potato layer. Potatoes, butter, milk, sour cream, salt and pepper.
- Mushrooms. Standard button mushrooms or any variety of mushroom you'd like.
- Caramelized onions. Made with two large onions - these cook down to almost nothing.
How to make the Mushroom, Caramelized Onion, and Mashed Potato Tart
⏱Timing tip! While waiting for the pie dough to hydrate, make the caramelized onions, mushrooms, and mashed potatoes. These can also all be made a day in advance and assembled and baked before serving.
STEP 1: Make the all-butter pie crust and allow it to hydrate in the fridge for 2 hours.
STEP 2: Thinly slice the onions and caramelize over low to medium low heat, stirring occasionally until a deep golden color is formed. This takes about an hour. Transfer to a bowl.
STEP 3: Clean and quarter the mushrooms, then add to a very hot pan with oil and toss to coat. Allow them to sit for one minute, then toss again and sit. Allow the mushrooms to develop a deep golden color. Transfer to a bowl.
STEP 4: Peel and evenly chop the potatoes, then add to boiling water and cook until a sharp knife inserted goes in and out easily. Drain, then add the butter, milk, sour cream and seasonings and mash until smooth. Cool completely.
STEP 5: Roll out the pie dough on a lightly floured surface, then transfer the dough to a cast iron skillet or a sheet pan lined with parchment paper.
STEP 6: To the center of the dough (leaving 2-3" around the edges), spread on the mashed potatoes and top with dollops of goat cheese.
Then, add on the mushrooms, caramelized onions, and fresh thyme leaves.
STEP 7: Fold the pie dough edges towards the center, gently creasing areas as needed. Whisk together an egg with a small splash of water then brush the egg wash onto the exposed dough. Bake in a 375℉/190℃ for 45-55 minutes until the pastry is golden, then cool for about 15 minutes. Slice and serve!
Storing and Freezing
Store the potato and onion tart covered in the fridge for 3-5 days. Reheat the tart or slices in the microwave, or in a 350℉ oven until the mashed potato layer is warm.
This tart should not be frozen.
FAQs & Tips
I would not suggest doing so for this recipe. The all-butter pie crust rolls out to a much larger piece of dough than store bought, which is needed to hold all of the delicious fillings.
Thankfully, this recipe actually doesn't even require a tart pan. You only need a cast iron skillet or large baking pan.
More savory recipes to try
Mushroom, Caramelized Onion and Potato Tart
- Rolling Pin
- 10" cast iron skillet or large baking sheet
- Pastry brush for egg wash
- 1 disc pie dough using this all-butter pie crust
- 2 large yellow onions peeled and thinly sliced
- ½ tablespoon extra virgin olive oil
- 2 russet potatoes peeled and evenly chopped
- 2 tablespoons salted butter 28g
- ¼ cup sour cream 57g
- 2 tablespoons milk 30ml
- ½ teaspoon kosher salt
- ¼ teaspoon black pepper
- 8 oz mushrooms of choice quartered or roughly chopped
- 1 tablespoon olive oil
- 2-4 oz goat cheese
- 2 sprigs fresh thyme
- 1 egg for egg wash
- Make the all-butter pie crust following recipe instructions and hydrate in the fridge for 2 hours.
- Remove the onion peels and thinly slice each onion, discarding the root.
- Heat a large skillet over medium high heat and add the olive oil. Then, add the sliced onions and toss to coat.
- Reduce the heat to low and stir occasionally. The onions will begin to get translucent, then start to gain color, then turn to a nice caramelized color and texture.
- Increase or decrease the heat, as needed, taking care not to burn the onions. This takes about an hour. Transfer to a bowl to cool.
- Clean and quarter the mushrooms. Then, heat a large pan over medium high heat and let it get very hot. Add the olive oil and mushrooms and toss to coat, then spread them evenly in the pan.
- Allow the mushrooms to cook for about a minute, then toss. Cook until browned and any liquid released has cooked off. Transfer to a bowl to cool.
- Add water to a large pot and bring it to a boil. Peel and chop the potatoes into even pieces, about 2" chunks.
- Once boiling, salt the water liberally, then add the potatoes. Cook for 15-20 minutes, or until a sharp knife inserts into the center of the potatoes without any give.
- Drain, then add back to the pot over low heat. Partially mash the potatoes, then add the butter, sour cream, milk, salt and pepper and continue mashing until smooth. Transfer to a bowl to cool.
- Preheat the oven to 375℉/190℃ and parchment line a cast iron pan or large cookie sheet.
- Lightly flour your work surface and roll out the pie dough until it's between ⅛-¼" thick, then drape it into the cast iron pan (or place on a sheet pan).
- Spread the mashed potatoes evenly into the center (meeting the edges of the cast iron), leaving 2-3" of the dough bare. Sprinkle the goat cheese on top of the mashed potatoes, followed by the mushrooms, caramelized onions, and thyme leaves.
- Fold the excess pieces of pie dough in towards the center in 4-5 sections, gently pressing down.
- Whisk together the egg and water, then brush it completely over the top of the pie dough.
- Bake the tart for 45-55 minutes until the crust is golden in color. Remove from the oven and cool for about 15 minutes, then slice and enjoy!