Thaw pastry sheets and lay them on a large cutting board.
Preheat oven to 400 and grease a muffin pan. (24-cup muffin pan or 2 of the 12-cup ones.)
Preheat a large cooking pan over medium-high heat. Add oil.
Dice onion and garlic and saute until onion is transparent.
Dice mushrooms and add them to the pan. Season with some salt. Saute over medium-high heat until done. Drain off liquid and transfer mushrooms to the mixing bowl.
Let mushrooms cool a little bit and then, mix in grated white cheddar cheese. Mix well.
Roll puff pastry a little bit (just a little) and cut the sheets into 9 squares each.
Place each square into the muffin cup, forming a little cup out of the pastry square.
Fill each pastry square with about 1 1/2 tablespoons of mushroom cheese mixture.
Bake for 15-17 minutes, until pastry is golden-brown.