Mix sauce. In a large mixing bowl, whisk together the sour cream, mayonnaise, ranch seasoning mix, garlic, onion powder, salt, pepper, Parmesan cheese, and ⅓ cup chopped green onions until smooth.
Assemble. Spread a thin layer of the sour cream mixture across the bottom of a 9×13 baking dish. Arrange the chicken breasts on top, then spoon the remaining sauce evenly over the chicken.
Add topping. Crush the crispy fried onions into coarse crumbs using a food processor or a zip-top bag and rolling pin. Sprinkle evenly over the chicken.
Bake. Bake for 45 to 60 minutes, until the chicken reaches an internal temperature of 165°F.
Finish. Let rest for a few minutes, garnish with the remaining green onions, and serve warm.
Notes
Chicken options. Chicken tenders or boneless thighs can be used in place of breasts.
Dairy swap. Plain Greek yogurt works well instead of sour cream.
Pan size. Recipe can be halved and baked in an 8×8 dish with the same bake time.
Storage. Store leftovers in an airtight container in the fridge for 3 to 5 days.
Reheating. Reheat in the microwave for 2 to 3 minutes or in a 350°F oven for about 15 minutes.