Make cheese sauce: melt butter, whisk in flour, cook 1 min. Gradually add milk, cook until thickened 5-7 mins. Stir in remaining seasonings, half mozzarella, Parmesan.
Combine potatoes and chicken in dish. Pour cheese sauce over, sprinkle remaining mozzarella. Bake 20-25 mins until golden. Rest 5 mins, garnish with parsley.
Notes
For extra flavor, add a pinch of red pepper flakes or herbs.