3poundsrusset potatoes, about 4-5 medium, peeled and sliced 1/8-inch thick
1teaspoonsalt
4tablespoonsbutter
1mediumonion, chopped
1teaspoondried thyme
3mediumgarlic cloves, finely minced
6tablespoonsall-purpose flour
2cupslow-sodium chicken broth
¾cupheavy cream or milk
6ouncesSwiss cheese, shredded, about 2 cups
2tablespoonsDijon mustard
¼teaspooncayenne pepper
2poundsboneless skinless chicken breasts, trimmed of excess fat and sliced thinly
9ouncesdeli ham, chopped
2tablespoonsminced fresh parsley leaves
Instructions
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). Sprinkle with parsley and cool for 10 minutes before serving.
Notes
Make-Ahead Tip: I made the entire casserole the morning before I wanted to serve it and stored it in the fridge all day. It cooked up beautifully. The original recipe recommends making the topping and cheese sauce and then refrigerating them in separate bowls for up to 1 day. To reheat and assemble, poke several vent holes in the plastic wrap covering the sauce and microwave it for about 3 to 5 minutes, until it is hot. Cook the potatoes and assemble and bake according to the recipe directions.