Add sliced Sunions and cook until reduced and caramelized -- approximately 25 minutes, stirring occasionally. Do not let them burn. Set aside.
Roll pastry into a 9x9 square on a baking sheet lined with parchment paper. Prick with fork to prevent the pastry from forming bubbles while baking.
Spread Sunions over pastry, leaving a half inch of pastry exposed around the edges. Dot with cheeses. Sprinkle with tarragon.
Bake in preheated oven at 400° for 18-20 minutes or until cheese has melted and pastry is crispy. Let cool for 5 minutes. Refrigerate if not baking immediately.