6Tbspunsalted butter, in small piece -- plus some to grease foil
FOR THE FILLING
2mediumshallots, finly sliced
1tspfresh thyme, minced
1½poundmushrooms, thinly sliced
freshly ground pepper to taste
¼cupMascarpone cheese, at room temperature
½cupaged Gouda or Gruyere cheese, freshly grated
¼cupParmesan cheese, freshly grated
FOR THE CRUST
Put the flour, cornmeal and salt into the bowl of a food processor. Add in the butter and pulse until the butter is in tiny bits. Add the egg and process until the dough starts to clump together. It will look like wet sand.
Dump the dough onto a floured surface and use your hands to bring it together. Roll out the dough to a 12-inch circle, rolling out the dough between two pieces of waxed paper.
Transfer the dough to a 9-inch tart pan and press the dough against the bottom and sides. Remove any excess dough from the top. Place the tart pan on a baking sheet and put it in the freezer until firm, about 30 minutes.
After chilling: Preheat the oven to 375˚F. Lightly butter one side of a 12-inch square of aluminum foil. Press the foil firmly against the bottom and sides of the crust, butter side down. You don’t need to use pie weights.
Bake for 10 minutes, then carefully remove the foil and bake another 5 to 8 minutes, until the crust is lightly golden brown around the edges. Set aside on a wire rack until the filling is ready. Reduce the oven temperature to 350˚.
MAKE THE FILLING
Heat a large skillet over medium heat. Add the olive oil and butter. When the butter is melted, add the shallots. Cook, stirring occasionally, 2 to 3 minutes, until the shallots are soft. Add in the garlic and thyme, and cook until fragrant, about 30 seconds. Increase the heat to medium high and add in the mushrooms. If the mushrooms won’t all fit in your skillet, add as many as you can, and as they cook down you’ll be able to add more to the pan.
Cook about 9 to 10 minutes, stirring occasionally, until the mushrooms are tender and the liquid has evaporated. Stir in ½ teaspoon salt and pepper to taste. Transfer to a plate to cool.
Put the Mascarpone in a medium bowl. Slowly pour in the milk, and whisk until the mixture is smooth. Whisk in the eggs. Add in the grated cheeses and remaining ½ teaspoon salt. Add in the cooled mushrooms and mix to combine.
BAKE THE TART
Carefully pour the mixture into the parbaked tart shell. Bake for 35 to 40 minutes, until slightly puffed and lightly golden brown on the top. If you insert the tip of a small knife into the center, it should not release any wet custard. Let the tart cool for at least 10 minutes before removing from the pan to serve. Serve warm or at room temperature.