Thinly slice the onions and heat a large saucepan over medium-low heat. Melt the butter and add the sliced onions. Stir occasionally, and cook until onions are soft, caramelized, and a nice brown color. If frond is sticking to the bottom of the pan and won't come up with stirring, add a small splash of balsamic vinegar (or chicken stock or white wine), and deglaze the pan. Onions are done when they are very soft and sweet, about 40 minutes to an hour.
2 large onions, 1 T butter, balsamic vinegar, white wine, or chicken stock
Prep
Preheat oven to 425 degrees F.
Take puff pastry out of freezer and set aside to thaw.
2 sheets puff pastry
Filling
Chop the bacon and cook over medium heat until crispy.
5 slices thick cut bacon
Quarter (or cut into sixth for larger pieces) the mushrooms and saute in 1 T of left over bacon grease (or regular oil such as canola) over medium heat.
10 crimini mushrooms
When softened, add balsamic vinegar and dried thyme and stir, cooking for a minute or two longer.
1 t balsamic vinegar, 1 t dried thyme
Pastry
When puff pastry is sufficiently defrosted, unfold and lay out on a cutting board.
Cut each sheet into 6 squares. Taking a paring knife, cut another square inside each square, about 1/2 inch in from each side. We want to set the filling on the smaller square and have the sides puff up around it.
Beat the egg with a splash of water and using a pastry brush, brush the egg wash onto the puff pastry (you can just do the outer portion since the middle will be covered with the onion mixture).
1 egg
Assemble
Combine caramelized onions, bacon, and mushrooms in a bowl, and spoon onto the centers of each puff.
parmesan cheese
Notes
Feel free to change the amounts of each ingredient according to taste.