Directions
Step 1
Preheat oven to 350°. Coat a 13"-by-9" baking dish and a rimmed baking sheet with cooking spray.
Step 2
Arrange prosciutto on prepared baking sheet in an even layer with no overlap. Bake until crispy, 10 to 15 minutes. Let cool (they will continue to crisp while cooling).
Step 3
Meanwhile, in a large saucepan over medium heat, melt butter. Whisk in flour and let bubble until light golden brown and toasted, about 2 minutes. Pour in broth and milk and simmer, whisking occasionally, until thickened, 4 to 6 minutes. Whisk in mustard, salt, and pepper.
Step 4
Stir chicken and ham into sauce. Simmer, stirring occasionally, until chicken is cooked through, 9 to 11 minutes. Stir in 2 cups cheese and immediately transfer mixture to prepared baking dish. Top with remaining 2 cups cheese.
Step 5
Place cooled prosciutto in a small bowl. Using the handle of a wooden spoon, crush into fine crumbs. Add panko and toss to combine. Sprinkle prosciutto mixture evenly over baking dish.
Step 6
Bake until filling is bubbling and crumbs are golden brown, 30 to 35 minutes. Top with parsley and serve.
