In a small bowl, stir together the yogurt and 1 tablespoon onion soup mix.
In another bowl, stir together 1 tablespoon onion soup mix, olive oil, mustard, thyme and mushrooms.
Unfold the pastry dough and allow it to thaw. Roll out slightly and cut each sheet into four pieces. Fold the edges into the center (about 1/2 inch). Press down with a fork, and pierce the bottom with a fork.
Evenly divide the toppings in this order: 1) Spread the Greek yogurt mixture on the bottom.
2) Add a few slices Brie cheese.
3) Add mushrooms.
4) Sprinkle with Parmesan cheese.
Bake in a 400-degree oven for 15-18 minutes, until golden. Remove from oven and sprinkle with kosher salt and fresh ground black pepper. Serve warm.