Caramelized Onion & Fennel Tart with Rosemary Crust

This filling tart is hearty enough to be the main meal at your next holiday gathering. The dough can be made up to three days ahead and refrigerated, wrapped in parchment paper, until you’re ready to roll it out. The caramelized onion-fennel mixture can also be made a day in advance, so you just have to assemble the components and pop the tart in the oven on the day you want to eat it. Slice the onions and fennel as thinly as possible; they’ll cook faster. Check them often while caramelizin