Preheat oven to 350°F. Lightly grease a 9x13” baking dish.
Build the base: Spread cream of onion soup into the dish. Pour in ½ cup white wine and scatter the sliced red onions and 1 tbsp minced garlic over the top.
Prepare the chicken: Butterfly 3 chicken breasts into about 6 thin cutlets. Pat dry and season both sides lightly with salt and pepper. Lay over the wine-onion base.
Mix the topping: In a bowl, combine 1 cup mayonnaise, Pecorino Romano, Italian dressing mix, and garlic.
Spread it on: Coat the tops of each chicken cutlet with the mayo mixture.
Add final toppings:
Sprinkle extra Pecorino Romano on top.
Drizzle a light splash of white wine over everything.
Sprinkle breadcrumbs generously over the mayo-coated chicken.
Cover the baking dish tightly with foil.
Bake at 350°F for 1 hour, covered.
Uncover the dish, switch the oven to broil, and let it broil for 2–4 minutes, until the top is golden and slightly crisp.
Serve hot, garnished with fresh parsley and more cheese if desired!