Beat cream, cheese, butter and flour until smooth and chill for at least 1 hour.
Add the mushrooms to the bowl of a food processor and pulse until finely chopped and transfer to a large skillet. Add the butter to the skillet and cook over medium high heat, until the mushrooms give off some of their liquid.
Add the onion to the food processor and pulse until finely chopped. Add onions to the mushroom mixture. Cook until the onions have cooked and softened, about 5-7 minutes.
Stir in the sour cream, salt, thyme and flour. Cook for about a minute until thickened and remove from heat to cool.
Meanwhile, roll out the dough and cut into 2 3⁄4 in circles (or thereabouts) and press into a mini muffin pan.
Fill pastry cups with 1 teaspoon of mushroom filling. Bake at 450 degrees F for 12-14 minutes or until the crust is a light golden brown.
Can be made ahead and frozen until ready to serve. To reheat tartlets, defrost them for 20-30 minutes and warm in a 325 degree oven for 8-10 minutes.