Spray a 10-inch skillet with the cooking spray and heat over medium-high heat for 1 minute. Add the chicken and cook for 10 minutes or until it's well browned on both sides.
Add the stock, garlic and broccoli to the skillet and heat to a boil. Reduce the heat to low. Cover and cook for 5 minutes or until the chicken is cooked through. Remove the chicken from the skillet and keep warm.
Stir the flour, milk, mustard and black pepper in a small bowl until smooth. Gradually stir the flour mixture into the skillet. Increase the heat to medium-high. Cook and stir until the mixture boils and thickens. Serve with the chicken and noodles.