Preheat the oven to 350 degrees F. In a medium skillet, set over medium heat, add the oil. When hot, add the fennel and sear on both sides, for about 1 to 2 minutes per side. You may need to do this in batches depending on how big your skillet is. See note below for slicing the root end off each slice. After you’re done with the fennel, transfer it to a food processor.

In the same medium skillet, add a teaspoon of olive oil, if needed. Add the leeks and cook, mixing often, until softened, about 2 to 3 minutes. Mix in the minced garlic and cook until fragrant, about 1 minute. Transfer the leek mixture to the food processor, along with the brie cheese, Parmesan-Reggiano, lemon zest, salt and crushed red pepper. Pulse until mostly smooth, about 30 seconds. Give it a taste and adjust the salt to your liking.

Transfer to a small oven-safe baking dish or bowl. Top with the shredded mozzarella and place in the oven until the cheese is melted and browned on top, about 15 to 20 minutes. Serve with slices of bread or crackers.