If you’d like the top a little more golden, you can switch the oven to broil for 2 to 3 minutes at the end, watching closely so the sour cream and onions brown but do not burn.
Remove the casserole dish from the oven and let the chicken rest for about 5 minutes. This helps the juices settle and the sauce thicken slightly.
To serve, spoon the caramelized onions and tangy sour cream sauce over each chicken breast. Serve straight from the casserole dish at the table for a cozy, family-style birthday (or any night) dinner.
For picky eaters, you can scrape most of the onions to one side of the dish before serving and give them just the saucy chicken, or even bake one chicken breast in a small separate dish with only sour cream on top. If your family likes a little extra flavor but you still want to keep it simple, stir a packet of dry onion soup mix or a teaspoon of garlic powder into the sour cream before spreading it over the chicken. For a slightly lighter version, use light sour cream and smaller chicken cutlets; just start checking for doneness a bit earlier. If you prefer dark meat, substitute bone-in, skinless chicken thighs and add 10 to 15 minutes to the baking time, or until they reach 175°F and are very tender. To make it feel more like a restaurant dish for a birthday dinner, sprinkle a little fresh chopped parsley or chives over the top right before serving and bring the whole casserole to the table on a trivet. Leftovers reheat well; slice the chicken and warm it gently in the sauce, then serve over toast or rice for an easy next-day lunch.