In a large skillet, set over medium heat, add half the Olive Oil (1 tablespoon). When hot, add the Fennel Bulb (2) and sear on both sides, for 1-2 minutes per side. After you’re done with the fennel, transfer it to the bowl of your food processor.
In the same skillet, add the remaining Olive Oil (1 tablespoon). Add the Leek (2) and cook, mixing often, until softened, 2-3 minutes.
Mix in the Garlic (2 clove) and cook until fragrant, about 1 minute.
Transfer the leek mixture to the food processor along with Kosher Salt (1/2 teaspoon) and Crushed Red Pepper Flakes (1/4 teaspoon). Give it a few quick pulses just to break them down and incorporate the ingredients.
Add the Brie Cheese (7 ounce) (without the rind), Parmigiano-Reggiano (1/2 cup), and zest of the Lemon (1 teaspoon). Pulse until mostly smooth, about 30 seconds. Check for seasoning and add salt if desired.
Transfer to a small oven-safe baking dish. Top with the Shredded Mozzarella Cheese (1/4 cup) and place in the oven until the cheese is melted and browned on top, about 20-25 minutes.