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.
Chicken Mushroom 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.
Cream Cheese Topping
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.