1In a large bowl, knead together, by hand, the butter, cream cheese, and flour.
2Divide the dough into two disks, wrap in plastic wrap, and place in the refrigerator for at least an hour.
3In a large saucepan, over medium-high heat, melt the butter. Add the mushroom and onions, and reduce heat to medium. Cook for 15-20 minutes, or until all the liquid has evaporated. Add the garlic, flour, salt, pepper, and thyme. Cook for a minute or two more. Transfer the mixture to a bowl, let it cool a bit, and then mix in the sour cream.
4Roll out the dough to 1/8 inch thick. Use a 3-inch round cutter to cut out as many rounds as possible. Form the scrap dough into a disk and re-refrigerate so it may be used again. Repeat with the remaining dough to make about 4 dozen rounds. Preheat the oven to 375 degrees (F). Line a baking sheet with parchment paper or use a Silpat mat.
Place 1 teaspoon of filling on each dough round and a pinch of cheese, brush edges with egg wash. Fold over to make a half circle shape. Crimp with the tines of a fork.
5Brush the tops with egg wash. Bake at 375F for 10-15 minutes, or freeze on the baking sheet and remove to a bag once frozen. If you bake them from frozen, set your oven to 450F and bake for 12-14 minutes.