Thaw the pastry sheet at room temperature for 40 minutes or until it's easy to handle. Heat the oven to 400 degrees F. Line a baking sheet with parchment paper or spray with vegetable cooking spray.
Heat the oil in a 12-inch skillet over medium heat. Add the onion and cook for about 15 minutes or until golden, stirring often. Season to taste and let cool.
Unfold the pastry sheet on a lightly floured surface. Roll into a 12-inch square. Place on the prepared baking sheet. Brush the edges of the square with water. Fold over the edges 1/2 inch on all sides, pressing firmly with a fork to form a rim. Prick the pastry thoroughly with a fork.
Spread the onion mixture over the pastry crust. Top with the gruyere and parmesan cheeses and sprinkle with the chives.
Bake for 20 minutes or until golden and bubbly. Cool for 5 minutes on a wire rack. Cut into approximately 3 x 2-inch rectangles. Serve warm.