Preheat the oven to 350 degrees F. Lightly grease a 9X13-inch baking pan.
For the sauce, in a medium saucepan, bring the broth and all the seasonings to a simmer. In a liquid measuring cup or small bowl, vigorously whisk the milk and flour together until smooth. While whisking the broth, slowly pour in the flour/milk mixture, whisking constantly. Continue to stir and cook until the mixture bubbles and thickens, 3-4 minutes. Remove from the heat and add additional salt and pepper to taste as needed.
Pat the chicken dry and lightly season with salt and pepper on each side. Place the chicken in a single layer in the baking pan - it's ok if the chicken is crammed in side-by-side as long as it isn't overlapping much.
Place a slice of Swiss cheese on every chicken breast. Pour the sauce evenly over the top. Cover the pan with aluminum foil and bake for 30 minutes.
While baking, make the topping: in a ziploc bag or in a bowl, toss together the panko, poultry seasoning, garlic powder, onion powder and dried parsley. Drizzle in the melted butter and mix to combine.
After the first 30 minutes of baking, remove the pan from the oven, uncover and sprinkle with the topping mixture. Return to the oven, uncovered, and bake for 20 minutes, until the crumbs are golden and the chicken is cooked through.
Let the casserole rest 5-10 minutes before serving. Serve over hot, cooked rice or quinoa, if desired.
Notes
For Serving: this is wonderful served over hot, cooked rice or quinoa. Sauce: I find more and more that I like to double the sauce for this recipe so if you like things a little on the saucier end, you might want to do the same!To Freeze: let the sauce cool before assembling. Follow the directions in the recipe but instead of topping the casserole with the panko mixture, put the panko mixture in a freezer ziploc, then cover the casserole dish with a double layer of foil. Freeze both the topping mixture and the casserole. For baking, put the frozen casserole, covered, in the preheated oven and bake for about an hour and 20 minutes. Uncover, top with the panko mixture and continue to bake for another 30-45 minutes until the chicken is cooked through and the crumbs are golden.