Step 1
Preheat the oven to 350 degrees F (175 degrees C). Spread hazelnuts onto a baking sheet.
Step 2
Toast in the preheated oven until golden brown and fragrant 10 to 15 minutes. Let cool.
Step 3
Transfer hazelnuts to a food processor and pulse until finely ground. Add flour and salt; pulse to combine. Add butter pieces 1 at time pulsing in between until mixture has a coarse grainy texture. Add egg yolk and pulse once. Pour in iced water 1 tablespoon at a time pulsing once or twice after each addition until you can pinch the dough together. Do not over-process.
Step 4
Turn dough out onto a work surface and form into a ball. Wrap in plastic wrap and refrigerate at least 30 minutes or up to 1 day.
Step 5
Melt butter in a large frying pan over medium-high heat. Saute shallot for 1 to 2 minutes. Add mushrooms and saute until tender and liquid has evaporated 5 to 7 minutes. Remove from heat season with salt and pepper and let cool completely.
Step 6
Preheat the oven to 375 degrees F (190 degrees C). Line a 10-inch tart pan with a removable base with parchment paper.
Step 7
Remove dough from refrigerator and let sit for a few minutes. Roll out into 12-inch circle on a lightly floured work surface. Carefully place dough into the prepared tart pan. Roll a pin over the top of the dough to trim it. Gently crimp the dough rim along the fluted edge of the pan. Line the tart shell with a piece of aluminum foil and fill with pie weights or dried beans.
Step 8
Bake in the preheated oven until set to the touch 8 to 10 minutes. Remove the weights and foil; continue baking until shell is golden about 5 minutes more. Transfer to a rack to let cool. Reduce oven temperature to 350 degrees F (175 degrees C).
Step 9
Place chevre cheese in a bowl and lightly beat with a wooden spoon until mostly smooth. Add eggs and beat to combine. Mix in half-and-half until smooth. Season with salt and pepper.
Step 9
Pour cheese mixture into the pastry shell. Scatter the cooled mushrooms evenly on top gently pressing them in. Sprinkle parsley on top.