Upside-Down Cream Of Mushroom Tartlets

1 . Stage

Preheat oven to 375° and line 2 baking sheets with parchment. Slice 4 oz mushrooms into 1/4" slices; set aside.

2 . Stage

In a food processor, pulse shallot and remaining 4 oz. mushrooms until finely chopped, about 15 times.

3 . Stage

In a medium skillet over medium-low heat, heat oil. Add shallot mixture; season with salt. Cook, stirring occasionally, until light brown and most of the liquid is evaporated, about 15 minutes. Transfer mushroom mixture to a medium bowl and cool slightly.

4 . Stage

Add Gruyère, heavy cream, sour cream, cornstarch, and thyme and mix until thoroughly combined.

5 . Stage

On a lightly floured surface, lay pastry flat. Cut into 12 (3") squares. Place 2 slices reserved mushrooms directly on parchment on a prepared sheet. Top with 1 tbsp. filling. Arrange a pastry square on top of filling and gently press around the edges to enclose. Repeat with remaining tartlets, spacing about 2" apart (you should have 6 on each sheet).

6 . Stage

Bake tartlets until pastry is puffed and golden brown, 15 to 20 minutes. Let cool slightly, then invert onto a wire rack and let cool completely.

7 . Stage

Make Ahead: Filling can be made 3 days ahead.. Store in an airtight container and refrigerate. Puff pastry squares can be cut to size1 day ahead. Place on a parchment-lined sheet, tightly wrap with plastic wrap, and refrigerate.