In a large bowl, beat together the 8 ounces cream cheese, 1½ cups flour and ½ cup butter to form a dough. Chill for one hour.
Preheat oven to 450°.
FOR THE MUSHROOM FILLING:
Add 8 ounces mushrooms to the food processor and pulse until very finely minced. Transfer to a bowl.
Cut 1 large onion into large chunks and add them to the bowl of the food processor. Pulse several times to mince.
In a skillet over medium high heat, saute the mushroom and onion in 3 tbsp butter until tender.
Add the ¼ cup sour cream, 1 tsp salt, ¼ tsp dried thyme and 2 tbsp flour and stir to combine. Cook for one minute; remove from heat to cool.
FORMING THE TARTLETS:
Roll the dough to about ¼" thick on a floured surface and cut into rounds with a 2" biscuit cutter.
Fit the dough rounds into mini muffin tins and lightly tamp them into place with the dough tamper.
ASSEMBLING MUSHROOM TARTS:
Fill the pastry tart shells with one teaspoon of mushroom filling. Bake for 12-14 minutes until the pastry is golden brown. Serve.
TO MAKE AHEAD AND FREEZE:
Cool the mushroom appetizers to room temperature and place them on a baking sheet. Freeze the tarts until solid. Transfer the mushrooms tart to a storage container and freeze for up to a month—Defrost and reheat to serve.