- 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