Place the flour, butter and salt in the bowl of a food processor. Pulse a few times to break up butter into little pieces… mixture should resemble coarse corn meal. With the machine running, add the water a tablespoon at a time until the dough holds together when squeezed. Gather the dough together and form into a rectangle. Wrap in plastic wrap and refrigerate for one half an hour.
In the mean time, heat the butter and olive oil in a large sauté pan. Add the mushrooms, stir to coat, spread out in the pan and allow to cook, undisturbed for a couple of minutes. Stir once and redistribute, continue cooking in this same manor until mushrooms are golden brown. Remove from pan and set aside.
Add onions to the same sauté pan; add additional olive oil is necessary. Sauté until caramelized well, season with thyme, salt and pepper.
Divide pastry in half. Roll each piece of pastry into long strips, about 4 inches by 8 inches. Place on a parchment lined baking sheet and crimp the edge of the dough to create a rim. Cover the tart with parchment and fill with beans. Bake 15 minutes, remove the parchment and beans and bake another 5 minutes. Remove and allow to cool.
Reduce the oven to 350 degrees F.
Spread half of the cheese and half of the onions and half of the mushrooms on each tart. Bake for 15 minutes. Allow to cool 5 minutes.
To serve, cut each tart in half length wise and then into 6 equal pieces. Arrange on a serving plate.