Cook diced bacon over medium heat until crispy and browned, about 6-8 minutes. Remove and let it drain on a paper towel, keeping some bacon fat in the skillet.

Add sliced sweet onions to the skillet with reserved bacon fat. Season with salt and pepper, and cook on medium-low for 25-30 minutes until deep golden and caramelized. Add apple cider and cook for another 5 minutes.

Preheat oven to 400°F (200°C). Roll out the frozen puff pastry into a large rectangle, transfer it to a parchment-lined baking sheet, score a 1-inch border, and prick the center.

In a mixing bowl, combine ricotta cheese and pumpkin purée with a pinch of salt and pepper. Spread evenly within the scored border on the pastry. Layer on caramelized onions, mozzarella cheese, and crumbled bacon.

Sprinkle fresh sage leaves on top and brush the edges with an egg wash. Bake for 25-30 minutes until the crust is puffed, flaky, and golden.

Remove from the oven and sprinkle grated Parmesan cheese on top. Allow to cool slightly before slicing.