In a food processor, or by hand with a bench scraper, cut the cold butter into the flour and process. Add the ground walnuts and process for just a minute.

Next, add the Parmesan, egg yolk, water, and thyme and process quickly to combine. Flatten the dough into a 1” disc, wrap with plastic and refrigerate for 1 hour.

Preheat the oven to 375°F (191°C)

Remove and divide into 4 pieces. Spray your tart pans with a baking spray (e.g. Baker's Joy) or lightly coat them with shortening or butter. Roll the dough on a lightly floured surface and gently press them into the tart pans. You are looking for just under ¼” thickness on the tart shell.

Once formed into your tart pans, dock the dough with a fork (poke several holes in the bottom of the tart dough). The tart pan is shallow enough that docking the sides is not necessary.

Then cover each tart shell gently with plastic wrap and put pie weights or dried beans on top of the plastic to weigh the dough down. Due to the shallow nature of these tarts, metal pie weights work the best, but we use blackeye peas (over and over) for this and it works fine.

Put your tart shells on a sheet tray and “blind bake” your shells for 15 minutes. Remove the shells from the oven and carefully remove the pie weights/beans from each shell. Return the shells to the oven for 10 more minutes. Since oven temperatures/calibrations vary, keep an eye on your shells. They are done when they are golden brown. These shells can be removed from their tart pans when they have cooled and set aside until the filling is ready.

Clean your mushrooms and cut into pieces slightly larger than you would want in the final dish. Depending on your mushroom types, they may be in a variety of sizes, which is fine. Chop your shallot – this chop can be a small dice, not very fine.

Heat a large skillet on medium heat and add 1 tablespoon of the butter. Once the butter melts, add the shallots, and cook until translucent and soft. Add the other tablespoon of butter, turn the heat to medium-high or high, and add the mushrooms, 1 teaspoon of the salt, pepper, and thyme. Stirring often and flipping allow the mushrooms to absorb the butter and release their juices. The mushrooms will then cook in the buttery mushroom juice and then, as the liquid evaporates, start to brown.

Once almost all the liquid has evaporated and the mushrooms have browned slightly, reduce the heat to medium and add the wine. Cook until the wine is almost gone and remove from the heat. Allow the mixture to cook for a few minutes, then return to a medium-heat and add ¼ cup of cream and 1 teaspoon of salt. Stir often and cook until the mushrooms and cream have melded – about 3-4 minutes. Add the vinegar and cook for about 30 seconds. Once done, turn the heat to low while you complete the next step.

In a separate bowl, mix the mascarpone, 1 tablespoon of cream and the remaining ¼ teaspoon of kosher salt.

Spread a thin layer of the mascarpone mixture on the bottom and sides of each tart. With the mushroom mixture, fill the tarts on top of the mascarpone mixture, then sprinkle each one with the finely chopped parsley. If the tart shells are made ahead, they can be warmed in a 250° oven for 3-4 minutes and then filled with everything before topping with the parsley. These can also be enjoyed at room temperature.