How to Make It
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.