Preheat the oven to 400℉. Spray an 8x10-inch pan (or 8½ x 11 is okay too) with nonstick spray.
PREPARE THE SAUCE:
In a medium saucepan, whisk together the milk, flour, mustard, salt and pepper. Heat over medium-heat, stirring constantly, until the mixture comes to a low boil. Continue to stir until the sauce is thickened, about 3 to 4 more minutes. Remove from heat and whisk in the Swiss cheese. Stir until melted.
PREPARE THE MAIN DISH:
In a large bowl, combine the gnocchi, ham and peas. Add the sauce and stir to combine. Spoon the mixture into your prepared pan.
PREPARE THE TOPPING:
In a medium bowl, combine the breadcrumbs, cheese and butter. Sprinkle the topping over the gnocchi mixture in the pan. Cover with foil.
Bake, covered, for 10 minutes. Remove foil and bake an additional 15 minutes, or until the cheese sauce is bubbling and the topping is golden brown. Serve immediately!