Heat your skillet over medium heat and add the chopped bacon until it’s crisp. Remove the bacon and set it aside on paper towels to drain. Leave the rendered bacon fat in the skillet — this will deepen the flavor of the onions and tie everything together.
In the same skillet, add the sliced sweet onions with a pinch of salt and pepper and cook over medium-low heat for about 25–30 minutes, stirring occasionally until they’re golden and jam-like. Then pour in the apple cider, scrape the browned bits from the pan for extra flavor, and cook for another 5 minutes until soft and richly caramelized.
Preheat the oven to 400°F (200°C). On a lightly floured surface, roll out each thawed puff pastry sheet gently to smooth seams. Transfer to parchment-lined baking sheets. Score a 1-inch border around the edges (don’t cut through), and prick the center lightly with a fork to prevent over-puffing of the center.
In a bowl, mix the ricotta cheese and pumpkin purée together, seasoning with salt and pepper. Spread this evenly within the scored borders on the pastry. Top with caramelized onions, then sprinkle the freshly grated mozzarella and crumbled bacon over that. Leave a small margin near the edge to keep things crisp.
Scatter fresh sage leaves across the top of the tart. Whisk together the eggs and water for the egg wash and brush just the pastry edge for that golden rim. Bake in the preheated oven for about 25–30 minutes, until the pastry is puffed and golden. Rotate your baking sheet halfway through for even color. If the sage crisps too quickly, tent with foil for the last few minutes.
Once out of the oven, immediately sprinkle the tart with grated Parmesan cheese so it melts into salty goodness. Let it rest for about 5 minutes before slicing — this helps the layers settle and makes cleaner cuts. Serve warm or at room temperature.