1-First Step: Get the oven and baking dish ready Start by preheating your oven to 350°F. Lightly spray a 9×13-inch casserole dish with cooking spray so nothing sticks later. This simple step makes cleanup easier and keeps the casserole edges from grabbing onto the dish. While the oven heats, gather all your ingredients so the sauce comes together without scrambling around the kitchen.

2-Second Step: Build the bottom layers Spread the 5 1/2 cups shredded cooked chicken evenly across the bottom of the prepared baking dish. Try to keep the layer level so the sauce will soak in evenly. Next, scatter the 2 cups cubed ham over the chicken. This gives you the classic chicken cordon bleu flavor in every serving and helps the ham settle into the creamy sauce later.

3-Third Step: Make the cheese sauce Melt 4 tablespoons butter in a saucepan over medium heat. Once it melts, sprinkle in 1/4 cup all-purpose flour and cook for 1 to 2 minutes. You want it to look dry and clumpy, which means the raw flour taste has cooked out. Slowly whisk in 2 cups milk, keeping the mixture moving so it stays smooth. Now add 1 tablespoon Dijon mustard, 1 tablespoon lemon juice, 1/2 teaspoon garlic powder, and 1/4 teaspoon salt. Let the sauce simmer for 3 to 5 minutes, whisking constantly until it thickens. Keep the heat moderate so the milk does not scorch. When the sauce coats the back of a spoon, take it off the heat and stir in 1 1/4 cups shredded Swiss cheese and 1/4 cup grated Parmesan cheese. Taste it and adjust the salt if needed.

4-Fourth Step: Pour and spread the sauce Pour the warm cheese sauce evenly over the chicken and ham. Use a spoon or spatula to guide the sauce into the corners of the dish so every bite gets plenty of creamy coverage. If your sauce seems extra thick, you can gently spread it with the back of a spoon rather than trying to pour it in one spot.

5-Fifth Step: Add the Panko topping In a small bowl, mix 1 cup Panko bread crumbs with 2 tablespoons melted butter. Stir until all the crumbs are lightly coated. Sprinkle the mixture evenly over the casserole. This is the part that gives the dish its golden, crispy finish, so try not to leave any big bare patches.

6-Sixth Step: Bake until bubbly Bake the casserole uncovered for 30 to 35 minutes. You are looking for a bubbling center and a golden brown topping. If your oven runs hot, start checking near the 30-minute mark. The casserole should look hot throughout and lightly browned on top when done.

7-Final Step: Let it rest and serve Once the casserole comes out of the oven, let it cool for 5 to 10 minutes before serving. That short rest helps the sauce settle a bit, so scooping is easier and neater. Serve it warm with salad, broccoli, asparagus, or even over pasta or rice if you want something heartier.