Preheat the oven to 425 degrees.

Thaw the frozen Puff Pastry for about 30 minutes.Flour your surface and unfold the sheet.Prick all over with a fork.

Cut through the middle of the puff pastry lengthwise (you can also roll it to make a bigger square and then cut through the middle) and bake in the oven for 15 minutes until puff pastry is golden.

In a pan heat the butter and saute the leeks for about a minute.Add the mushrooms and cook until golden.

Add salt and pepper.Keep aside.

Spoon the vegetable mixture evenly all over the baked sheets and put it back into the oven for about 5-6 minutes.

Sprinkle feta all over.Cut into small rectangles/squares and serve warm or at room temperature!