Place the glass casserole dish on the middle rack of the preheated oven and bake for 30 to 40 minutes, or until the chicken is cooked through (the internal temperature should reach 165°F/74°C and the juices run clear). The sauce should be bubbly and the top lightly golden around the edges.
If you’d like a slightly more browned top, you can turn the oven to broil for 2 to 3 minutes at the end, watching closely so it doesn’t burn.
Remove the casserole from the oven and let it rest for about 5 minutes. This helps the sauce thicken slightly and keeps the chicken juicy.
Sprinkle with chopped fresh parsley or green onions, if using, and serve the smothered chicken hot, spooning extra creamy mushroom-Parmesan sauce from the glass dish over each piece.
For picky eaters who don’t love mushrooms, you can finely chop the mushrooms instead of slicing them so they blend into the sauce, or leave them out of half the mixture and mark one side of the dish as the ‘no mushroom’ side. If someone isn’t a fan of Parmesan, try using half Parmesan and half shredded mozzarella or mild cheddar for a creamier, less sharp flavor. To add more veggies while keeping it low carb, stir in a handful of fresh spinach or a few tablespoons of finely chopped bell pepper to the sour cream mixture before smothering it over the chicken. For extra flavor, sprinkle a little smoked paprika or Italian seasoning over the top before baking. If your chicken breasts are very large, you can slice them in half horizontally to create thinner cutlets; they’ll cook faster and more evenly, and it stretches the meal a bit further. Leftovers reheat well in the microwave or oven—just cover the dish so the sauce doesn’t dry out. If you need to prep ahead, you can assemble the raw chicken and rich sour cream-mushroom mixture in the glass casserole dish, cover tightly, and refrigerate up to 24 hours before baking; add 5 to 10 minutes to the bake time if going straight from the fridge to the oven.
