Boil potatoes until soft, about 20 minutes. Drain well and mash.

While they boil, heat oil in a large sauté pan. Sauté onions until limp.

Add mushrooms and sauté until all liquid evaporates.

Add mashed potatoes, mayonnaise, salt, pepper, and nutmeg. Mix until smooth.

Preheat oven to 375 degrees Fahrenheit.

Place one puff pastry sheet on a Silpat baking mat or floured work surface. Roll out to a 12 x 18-inch rectangle.

Using a jalousie roller, cut pastry as directed to yield 12 rectangles.

Working with one rectangle at a time, place one heaping tablespoon of filling on the solid end of the pastry.

Brush egg over the border.

Fold over the slitted end of the pastry. Press edges down with fingers to adhere.

Repeat with the other sheet of puff pastry and the remaining filling.

Bake 20 minutes, or until golden.