1-First Step: Get the oven and casserole dish ready Preheat the oven to 350°F. Spray a 9×13-inch casserole dish with cooking spray so the cheese sauce does not stick. This step sounds basic, but it saves cleanup later and helps the casserole release cleanly when serving. While the oven heats, gather all of your ingredients. Having everything measured out makes the sauce come together faster, which is helpful if you are cooking after work or while helping kids with homework.

2-Second Step: Build the chicken and ham layers Spread the 5 1/2 cups shredded rotisserie chicken across the bottom of the casserole dish. Then layer the 2 cups cubed ham over the chicken. This layering helps the sauce settle through the casserole and gives each scoop a nice mix of meat in every bite. If you are using leftover chicken, make sure it is cut or shredded into bite-size pieces. That way the casserole bakes evenly and serves neatly.

3-Third Step: Make the cheese sauce Melt 4 tablespoons butter in a saucepan over medium heat. Once melted, whisk in 1/4 cup all-purpose flour and cook for 1 to 2 minutes. This cooks out the raw flour taste and starts the thickening process. Slowly whisk in 2 cups milk, a little at a time, so the sauce stays smooth. Add 1 tablespoon Dijon mustard, 1 tablespoon lemon juice, 1/2 teaspoon garlic powder, and 1/4 teaspoon salt. Keep stirring, then let it simmer for 3 to 5 minutes until it thickens. Take the pan off the heat and stir in 1 1/4 cups shredded Swiss cheese and 1/4 cup Parmesan cheese. The sauce should be creamy and pourable. If it looks too thick, you can whisk in a splash more milk.

4-Fourth Step: Add the sauce and make the topping Pour the cheese sauce evenly over the chicken and ham. Use a spoon or spatula to spread it across the whole dish so every corner gets covered. This is what keeps the casserole moist while it bakes. In a small bowl, combine 1 cup Panko bread crumbs with 2 tablespoons melted butter. Stir until the crumbs are lightly coated. Sprinkle this mixture evenly over the top of the casserole. The buttery Panko gives the baked chicken cordon bleu casserole its crisp finish.

5-Final Step: Bake and serve Bake the casserole uncovered for 30 to 35 minutes, or until the breadcrumbs are golden brown and the casserole is bubbling around the edges. Let it cool for 5 to 10 minutes before serving so the sauce settles a bit. If you want, serve it over pasta or rice for an even heartier meal. It also tastes great with a side salad or a simple vegetable.