One partially baked and cooled 9-inch savory tart shell, purchase pre-made, or make from scratch. See my post on Tartine’s Flaky Tart Dough for instructions on how to make the dough from scratch, and also how to partially bake a tart shell.
1tablespoonfresh thyme, coarsely minced, for garnish
Equipment neededIf you have a 9-inch tart pan with fluted edges, use it, but if you don't, a 9-inch pie pan still works. Since a tart pan has a lower edge, the filling does not come completely to the top of the pie dish. To compensate, and make a larger tart, you can increase the recipe by a half to bring the filling to the top, and increase the cooking time. I have tried this, and it works.
Make the mushroom tart
Prepare the mushrooms by removing any stems you do not want to use, and slice or halve them. I like the pieces to be no more than a 1/2-inch thickness.
Melt the butter in a heavy saucepan (enameled cast iron if you have it), over medium-high heat. Add the shallots and sauté until they just start to brown. Add the prepared mushrooms, salt, and pepper. Sauté until the mushrooms are soft and starting to caramelize, about 10 minutes.
Add the lemon juice and water and use it to remove any caramelization (a.k.a. flavor) from the bottom of the pan. Remove from heat and set aside until needed. Preheat your oven to 350 degrees Fahrenheit.
In a mixing bowl, whisk together the crème fraîche, egg yolks, and nutmeg until smooth. Add the mushrooms and stir to combine. Turn the mixture into the partially baked tart shell.
Bake until the tart custard is barely firm to touch in the center. Although Tartine says it cooks in 20 minutes, it took 45 minutes to reach this stage in my oven. Remove from oven and allow to cool. The custard continues to set as it cools.
Serve warm, or at room temperature, sprinkled with fresh thyme. Use a serrated knife to cut pieces of tart with the crispy shell. Enjoy for lunch or dinner with a fresh green salad garnished with roasted beets, apples, goat cheese, and something like a red wine vinaigrette.
The type of mushrooms you use will affect the flavor of this tart. I prefer a combination of portobello, delicate and sweet-flavored oyster mushrooms, and mild-flavored brown cremini mushrooms. This selection is affordable and easy to find.If you want more mushroom flavor, and you don’t mind spending a little more, you could add some porcini, morel, or Chanterelles. I would not recommend Shiitake, as their flavor would be overpowering in this dish. Shiitakes are better for Asian cuisine. It is also a good idea to buy organic mushrooms if possible, because the fungi tend to soak up whatever they are grown in.Crème fraîche is a soured cream containing as much as 45 percent butterfat, with a pH of around 4.5. It is soured with a bacterial culture similar to sour cream, but it is comparatively less sour, thinner, and higher in fat. I used a high quality sour cream in this recipe and it worked well. You can purchase crème fraîche at most grocery stores.