Chicken Cordon Bleu Casserole

I substituted mozzarella in place of the swiss. This is GOOD!!!!

Chicken Cordon Bleu Casserole

Topping:
4 slices white sandwich bread, torn into quarters (I used a long forgotten bread bowl tucked in the back of the freezer - thawed it in the microwave before using)
2 tablespoons butter, melted
Filling:
3 pounds russet potatoes (about 4-5 medium), peeled and sliced 1/8-inch thick
1 teaspoon salt
4 tablespoons butter
1 medium onion, chopped
1 teaspoon dried thyme
3 medium garlic cloves, finely minced
6 tablespoons all-purpose flour
2 cups low-sodium chicken broth
3/4 cup heavy cream or milk
6 ounces Swiss cheese, shredded, about 2 cups
2 tablespoons Dijon mustard
1/4 teaspoon cayenne pepper
2 pounds boneless, skinless chicken breasts, trimmed of excess fat and sliced thinly
9 ounces deli ham, chopped
For the topping, process the bread and butter in a food processor until coarsely ground. Set aside.
For the filling, preheat the oven to 400 degrees, making sure the oven rack is in the middle. Place the potatoes, 6 cups water and 1 teaspoon salt in a large pot. Bring to a boil over high heat, then reduce the heat to a simmer and cook, uncovered, until the potatoes are partially cooked, about 2 minutes (don't overcook because they will continue to cook in the oven!). Carefully drain potatoes, or remove them with a slotted spoon, and transfer them to a bowl. Set aside.
Meanwhile, melt the butter in a medium saucepan over medium heat. Add the onion, thyme, and 1/2 teaspoon salt. Cook until the onions are softened, about 6 minutes. Add the garlic and cook until fragrant, stirring constantly, about 1 minute. Add the flour and cook, stirring constantly, until golden, about 1 minute (this helps cook out the flour taste in the roux). Slowly whisk in the chicken broth and the cream or milk. Bring the mixture to a simmer, whisking often, until slightly thickened (if using milk instead of cream, you may have to cook longer for it to thicken up a bit). Off the heat, whisk in the Swiss cheese, Dijon, and cayenne pepper until smooth. Season with salt and pepper to taste. Cover and set aside until ready to use.
Toss the potatoes with half of the sauce and spread into the bottom of a lightly greased 9X13-inch baking pan. Layer alternating pieces of the chicken and ham over the potatoes, overlapping as needed. This is not a scientific procedure, just make sure that the chicken is in a single layer (not on top of each other) so that it cooks thoroughly. Pour the remaining sauce evenly over the chicken and sprinkle with the bread crumb topping. Bake until the chicken is cooked through and the casserole is bubbling, 30 to 35 minutes (add about 5-7 additional minutes onto the baking time if baking it after it has chilled in the refrigerator for more than a couple of hours). Cool for 10 minutes before serving.