Prepare the gnocchi by making a homemade version or boil the store-bought variety by following package directions. If you are making your own gnocchi, you should double the recipe. Place gnocchi in a casserole dish.
Preheat the oven to 375°F.
Make the béchamel: In a large saucepan, melt 3 tablespoons of the butter over medium heat. Add the flour and stir with a wooden spoon until thick, around 30 seconds.
Add the milk, mustard, salt, pepper and nutmeg and stir until bubbling and thick, around 5-7 minutes.
Add the cheddar, mozzarella and American cheese and stir to combine.
Pour the sauce over the gnocchi.
Combine the parmesan and bread crumbs and sprinkle over the gnocchi and béchamel. Dollop the remaining 2 tablespoons of butter over the crumbs.
Baked uncovered for 30-35 minutes, until the top is golden brown. Enjoy!