1-First Step: Get the oven and pan ready Start by preheating your oven to 350°F. Grease a 9×13-inch casserole dish so the chicken cordon bleu casserole does not stick. This also makes cleanup much easier, which is always a win on a busy night. While the oven heats, gather your ingredients and measure everything out. That way, once the sauce starts cooking, you can move fast and avoid lumps. If you are using leftover chicken, make sure it is shredded or chopped into bite-sized pieces. Rotisserie chicken works great here.

2-Second Step: Layer the chicken and ham Spread the shredded cooked chicken evenly in the bottom of the prepared casserole dish. Next, layer the cubed ham over the chicken. This order helps every bite carry both meats and keeps the base sturdy under the creamy sauce. If you want to stretch the dish for a larger group, you can mix in cooked rice, pasta, or broccoli. That turns this chicken cordon bleu casserole into a bigger meal without changing the core flavor.

3-Third Step: Make the cheese sauce Melt 4 tablespoons of butter in a saucepan over medium heat. Stir in 1/4 cup of all-purpose flour and cook for 1 to 2 minutes until it looks clumpy. This is the roux, and it helps thicken the sauce later. Slowly whisk in 2 cups of milk, preferably at least 2% fat. Keep whisking so the sauce stays smooth. Then add 1 tablespoon Dijon mustard, 1 tablespoon lemon juice, 1/2 teaspoon garlic powder, and 1/4 teaspoon salt. Let it simmer for 3 to 5 minutes, stirring constantly, until the sauce thickens. Do not rush this part. A little patience here keeps the casserole creamy instead of watery. Once the sauce is thick, take it off the heat and stir in 1 1/4 cups shredded Swiss cheese and 1/4 cup grated Parmesan cheese. Stir until melted and smooth. Taste the sauce and adjust the salt if needed. For the best texture, use milk that is at least 2% fat. Skim milk can make the sauce thinner and less stable.

4-Fourth Step: Assemble the casserole Pour the cheese sauce evenly over the chicken and ham. Use a spoon or spatula to spread it out so the sauce reaches the corners of the dish. You want every bite to have a little sauce, a little meat, and a little crunch on top. If you are making this ahead, stop here and cover the dish without adding the topping. Keep it in the fridge until you are ready to bake. This makes the chicken cordon bleu casserole a strong choice for meal prep, company dinners, or church potlucks.

5-Fifth Step: Add the Panko topping In a small bowl, mix 1 cup Panko breadcrumbs with 2 tablespoons melted butter. Stir until the crumbs are lightly coated. Sprinkle the mixture evenly over the top of the casserole. This topping is what gives the dish that golden finish. If you like a little extra punch, you can add a bit more Parmesan or a pinch of paprika to the breadcrumb mixture. That gives the top a little more color and flavor.

6-Final Step: Bake and serve Bake the casserole uncovered for 30 to 35 minutes, or until it is bubbly and golden on top. The baking time may vary a little depending on your oven and how cold the casserole was when it went in. If you assembled it ahead of time from the fridge, it may need a few extra minutes. Let the finished chicken cordon bleu casserole cool for 5 to 10 minutes before serving. This helps the sauce settle a bit so it is easier to scoop. Serve warm and enjoy it with salad, roasted vegetables, pasta, or rice.