Preheat oven to 375°. In a large pot of salted boiling water, cook cavatappi until al dente. Drain and set aside.
In a large saucepan over medium heat, melt butter. Sprinkle over flour and cook until slightly golden, 2-3 minutes. Pour in milk and whisk until combined. Season with nutmeg if using, salt, and pepper. Let simmer until thickened slightly, about 2 minutes.
Remove pan from heat and stir in cheddar, Fontina, and 1/2 cup Parmesan and whisk until smooth. Fold in pasta and lobster and transfer into a 9"-x-13" baking dish.
In a medium bowl, combine Panko, the remaining Parmesan, and oil. Season with salt and pepper and sprinkle over pasta. Bake until bubbly and golden, 20-25 minutes.