The Upside Down Puff Pastry Croque Monsieur with Caramelized Blackberries is a savory twist on a classic French sandwich. Layers of ham, Gruyere cheese, and Dijon mustard sit atop a bed of caramelized blackberries and onions, all encased in crispy puff pastry. Once baked and flipped, the tart is garnished with fresh blackberries, chives, and a sprinkle of sea salt for a perfect balance of sweet and savory flavors.