tart shell

1. Combine the flour, salt and baking powder in a food processor and pulse a few times to combine. Add the butter and pulse until the mixture resembles a coarse meal and the butter is broken into pea-sized pieces, 15- 20 times. Alternatively, combine the dry ingredients in a large bowl and work the butter into the flour mixture with your fingers, knives or a pastry cutter.
2. Add the creme fraiche directly from the refrigerator and pulse about 10 times to mix and form a shaggy dough. Stop when no dry flour remains. If working by hand, mix in the creme fraiche with a fork until a dough begins to form, then knead in the bowl a few times until the dough comes together.
3. Place a 2 ft long sheet of plastic wrap on a clean surface and place dough on it. Cover dough in plastic wrap, form into a ball, then shape into a 6" x 10" rectangle. Smooth and flatten the dough with your hands to create a uniform shape. Chill in the refrigerator for at least 1 hour, or up to 24 hours.
4. Remove the dough from the refrigerator, unwrap, and let it rest until just soft enough to roll without cracking, no more than 15 minutes. On a floured work surface, whack the dough a few times with a rolling pin to soften it, then add more flour to the surface if needed.
5. Roll out the dough to form an 8"x 12" rectangle, with the long edge running parallel to the countertop's edge. Fold the rectangle in thirds, starting with the right edge and ending with the left edge. Rotate the dough 90°, so the left edge is now parallel with the countertop's edge. Roll out the dough to form an 8"x12" rectangle and again fold it in thirds, starting with the new right edge and ending with the new left edge. Rotate the dough 90°, so the left edge is now parallel with the countertop's edge. This time, roll out the dough to a 9"x15" rectangle that's about 1/4" thick, with the long edge running parallel to the countertop's edge. As you roll out the dough, if the butter gets too warm (leaks out or begins to stick), place it in the freezer for a few minutes to re-chill, then continue where you left off.
6. Cut the rectangle lengthwise into 3 strips, each 3” x 15”. Place the dough strips on a parchment paper-lined baking sheet and prick all over with a fork, leaving 1/4" border around the edge. Preheat the oven to 425°F and place the baking sheet in the freezer while the oven heats up, about 15 minutes.
7. Whisk together the egg and cream in a small bowl. Remove the pastry from the freezer and brush each strip with egg wash. Bake for 10 minutes, remove pan from the oven and reduce heat to 375°F. Wait a few minutes for the oven temperature to readjust, then bake the pastry sheets for another 12-15 minutes, or until they are a deep golden brown.
8. Set pastries aside to cool and turn the oven up to 450°F for the mushrooms.

roasted mushrooms

1. If not already on, preheat oven to 450°F.
2. Toss mushrooms with garlic, thyme sprigs, and olive oil, and season with salt and freshly ground pepper. Arrange on two foil-lined baking sheets so they are evenly spaced and not crowded.
3. Roast until mushrooms are golden and crisp, stirring two to three times, 25- 35 minutes. Discard garlic cloves and thyme sprigs, set aside.

filling

1. While the mushrooms roast, combine goat cheese, cream cheese and heavy cream in a small food processor fitted with a metal blade. Process until smooth and creamy. Alternatively, the cheeses and milk can be combined in a bowl with a handheld mixer
2. Add more cream, if needed, to make the mixture spreadable but thick enough so it doesn’t ooze off of the tart shell. Set aside.

assembly

1. Divide goat cheese filling between the baked pastry strips, spreading it evenly and leaving 1/4” of pastry uncovered around the edges (an offset spatula works great here). Scatter roasted mushrooms over the cheese filling and sprinkle with fresh thyme leaves.
2. Serve as is, or if desired, return pastries to the oven for 1- 2 minutes at 375 °F, until cheese is warm but not too runny and mushrooms are heated through. Cut crosswise into pieces and serve immediately.