Preheat oven to 350°F. Melt butter in a Dutch oven over medium. Gradually whisk in flour until smooth; cook, whisking constantly, 2 minutes. Whisk in dry mustard, salt, black pepper, and cayenne pepper. Gradually whisk in half-and-half and milk; cook, whisking constantly, until thickened, 8 to 10 minutes.
Whisk in Swiss cheese and 2 oz. Parmesan cheese. Remove from heat. Stir in pasta, ham, and peas. Pour mixture into a lightly greased 13- x 9-inch baking dish.
Prepare Topping: Process croutons and 2 oz. Parmesan cheese in a food processor until finely ground. Add 1 tablespoon melted butter, and process until combined. Sprinkle over pasta mixture.
Bake in preheated oven until golden, about 30 minutes. Let stand 10 minutes.
We tested with Hormel Cure 81 Premium Cubed Boneless Ham and New York Texas Toast Sea Salt and Pepper Croutons.