Preheat oven to 375°F. On a lightly floured piece of parchment paper, roll out puff pastry to an 11 x 14-inch rectangle. Trim edges to make a neat rectangle and transfer pastry on parchment to a rimmed baking sheet (trim parchment to fit, if necessary). With a sharp knife, score a 1/2-inch border around edges. With a fork, prick pastry all over inside border. Refrigerate 20 minutes.
In a large skillet, melt butter over medium. Add onion and cook, stirring occasionally, until soft and golden, 10 to 15 minutes. Add mushrooms and cook until mushrooms begin to soften, about 8 minutes; season with salt and pepper.
Place bare puff pastry in the oven and bake until light golden, about 15 minutes. Remove from oven and top with mushroom mixture and pancetta staying within border. Carefully crack eggs on top of mushroom mixture, spacing them evenly, and bake until pastry is golden and egg whites are set, about 10 minutes. To serve, sprinkle with Parmesan and cut into 6 squares.
Approximate Nutrient Analysis per serving (assumes 5 oz pancetta and not including salt to taste and grated Parmesan):
460 calories, 34 g fat, 10 g saturated fat, 210 mg cholesterol, 500 mg sodium, 23 g carbohydrate, 2 g fiber, 3 g sugar, 16 g protein