Make-Ahead Chicken Divan Casserole

Preheat the oven to 350 degrees F (175 degrees C).

Combine Cheddar cheese and 1/2 cup Parmesan cheese in a bowl and set aside.

Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add broccoli, cover, and steam until tender, 2 to 6 minutes.

Meanwhile, melt butter in a large skillet over medium heat. Add mushrooms, onion, and garlic; saute until soft, 5 to 7 minutes.

Whisk condensed soup, sour cream, milk, and mayonnaise together in a large bowl. Add 1/2 cup of the reserved Cheddar-Parmesan mixture and season with salt and pepper. Add the sauteed vegetables and stir to combine the sauce.

Spread 1/2 cup of the sauce onto the bottom of a 9x13-inch casserole dish. Add cooked rice as the bottom layer. Add cooked broccoli as the next layer, and then layer on the cooked chicken. Cover with remaining sauce. Pat down evenly and smooth with a spatula.

Bake in the preheated oven for 30 minutes. Sprinkle with remaining Cheddar-Parmesan mixture, plus extra Parmesan if desired, and continue to bake until browned, about 15 minutes more.