Preheat oven to 350°F (175°C) and spray a 9×13-inch baking dish with cooking spray.
Place the chicken breasts between two pieces of parchment paper and gently pound with a meat mallet or rolling pin until each breast is under 1 inch thick and even in thickness.
Season both sides of the chicken with salt and pepper, then arrange the breasts in the prepared baking dish. Top each breast with one slice of baby Swiss cheese.
In a small saucepan, whisk together the cold skim milk, flour, poultry seasoning, onion powder, and a pinch of salt and pepper until smooth.
Place the saucepan over medium heat and whisk constantly until the mixture thickens into a sauce, about 3–5 minutes; remove from heat and stir in the butter until melted.
Pour the hot sauce evenly over the cheese-topped chicken, then sprinkle the crushed reduced-fat Ritz crackers over each piece.
Cover the baking dish and bake in the preheated oven for 45 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and is cooked through.
Remove from oven, let rest briefly, then divide into four portions and serve hot.