Line the bottom of a 10-inch tart pan with parchment paper.Roll the puff pastry sheet out until it is about 14x14: big enough to fully line the tart pan up and over the sides.
Lay the rolled out puff pastry dough in the prepared tart pan (a shallow pie dish works well too) and trim the edges using the edge of a rolling pin rolled across the top.Put the pastry in the tart pan in the freezer while you prepare the filling.
Melt the butter and olive oil in a medium skillet over medium heat. Add onions and sauté until they lose their moisture and begin to become translucent; about 5-6 minutes.
Add sliced mushrooms and continue to cook until they release their juices; about 5-7 minutes. You can add a pinch of salt to the mixture to encourage the process.
When the mushrooms have cooked down some, add the chicken, chopped walnuts, rosemary, and garlic, and cook until any remaining liquid is absorbed.
Add the wine and deglaze the pan, continuing to cook up til the liquid has once again reduced. Season to taste with salt and pepper.
It is important that the filling be cooled before adding it to the pastry shell. To cool the filling faster, line a rimmed baking sheet with parchment paper and spread the filling across it. Pop it in the freezer for 5 minutes and it will be ready to use.
In a medium bowl, mix cream cheese, eggs, orange zest (optional), salt, and pepper together until smooth.
Remove the tart pastry from freezer and fill the shell with the cooled chicken mushroom mixture.Using a spatula, "frost" the top of the tart with the cream cheese mixture.
Bake for 30-35 minutes, until the top begins to brown and the egg mixture is firm.
Remove from oven and cool slightly before cutting. Tart can be served warm or cold.
Notes
Advanced Prep
All three tart components can be made up to a day ahead of time.
Prep the shell and freeze until you are ready to fill it.
Cook the filling and refrigerate it up to 24 hours.
Mix up the topping and refrigerate it up to 12 hours.
Storage
Store this mushroom tart in an airtight container or wrap it tightly in plastic wrap to prevent it from drying out or absorbing odors from the refrigerator. To prevent a soggy crust, place a paper towel between the tart and the storage container to absorb excess moisture.Properly store, this chicken mushroom tart can last about 4 days in the refrigerator. We do not recommend freezing this tart.