Instructions

1. Make the dough by putting the flour and salt into a large bowl and mixing. Make a well in the center of the flour and add the chilled butter, cut into small cubes. Using your hands, quickly work the butter into the flour, until the mixture resembles crumbs. Slowly add the cold water, mixing it until it comes together into a ball. If you are using whole wheat flour you will need a bit more water than usual, since it absorbs the water. Use the heel of your hand to work in the last bits of flour.
2. Divide the dough into two pieces, form it into two discs, wrap in plastic wrap and place in fridge for 30 minutes.
3. In the meantime clean the mushrooms. Cut off the bottoms if they are dirty and using a wet paper town, gently wash the mushrooms. You don’t want them to get too wet, or else they won’t brown when cooking. Cut them into 1/4 inch slices.
4. Trim and slice the onion or leek into 1/4 inch slices.
5. In a large frying pan, put 1 tablespoon of olive oil and heat over high heat. When it is hot, add the cardoncelli mushrooms (or portabella). RESIST THE URGE TO STIR. The trick with mushrooms is to let them cook and brown without stirring. They will begin to give up their liquid, reduce and brown. You can shake the pan a bit, but only stir at the end, when they have reduced. Season with salt and pepper and set aside.
6. Repeat for the cremini mushrooms.
7. To cook the onions place the 1 tablespoon of olive oil and butter into a large frying pan. Add the sliced onions, 1 teaspoon of salt, 1 teaspoons of thyme, and freshly ground black pepper. Cook over low heat until completely wiled and reduced. This may take up to 30 minutes. If the onions start to brown, add a bit of water.  When they are tender add the white wine, and let it cook off, about 3 minutes. Set aside.
8. To assemble the tart line a 10 inch tart pan with a removable bottom with parchment.
9. Preheat oven to 350F/ 180C
10. Remove the dough from the refrigerator and roll the first piece out into a disk big enough to cover the bottom of the tart pan. If it’s hard to roll out, let it warm up a bit. And if it starts to break up, don’t worry too much, you can patch it together.
11. Transfer the dough to the tart pan, letting the edges come up and over the sides.
12. Layer the grated cheese on top of the dough. Then spread the onions over that, followed by both kinds of mushrooms.
13. Rolls out the remaining dough and cover the tart, pinching the edges together around the rim.
14. Brush the top with egg yolk and place in preheated oven. Bake for 35 minutes.
15. Take out of oven and let cool for 10 minutes before serving. It is also very good at room temperature.