Preheat oven to 375 °F (190 °C). Grease a baking dish. Slice or pound chicken to 1″ thickness. Pat dry and season with salt and pepper.
Sear chicken in olive oil over medium-high heat, 2–3 minutes per side. Transfer to dish. (Skip if you prefer.)
Mix sour cream, mayo, garlic powder, onion powder, paprika, salt, pepper, and Parmesan in a bowl. Too thick? Add 1 tbsp milk.
Spread topping over chicken. Add mozzarella if using. Bake 35–40 minutes, until internal temp is 165 °F and top is golden.
Broil 2–3 minutes for extra color. Watch carefully.
Let rest 5 minutes. Garnish with parsley or chives. Add red pepper flakes if desired.