First, I set my oven to 375 °F (190 °C) and lightly spray a 9×13-inch baking dish. This gives my casserole a nonstick base and ensures even browning.
In a saucepan over medium heat, bring 1½ cups water and ¼ cup butter to a boil. Stir in the stuffing mix, remove from heat, cover, and let it absorb. Then fluff with a fork so it’s airy and not clumped.
In a skillet, melt 1 tablespoon butter, then add diced onion and celery. Cook and stir until they are tender and the onion turns translucent, about 5 to 10 minutes.
In a bowl, whisk together the cream of chicken soup, cream of mushroom soup, and sour cream until smooth and uniform. This becomes the luscious binder for the chicken.
Spread shredded rotisserie chicken evenly across the bottom of the prepared dish. Top with the onion-celery mixture, seasoning with salt and pepper. Then pour the soup mixture over that. Finally, spread the prepared stuffing on top.
Place the casserole in the oven and bake until the filling bubbles at the edges and the stuffing turns golden brown on top — about 1 hour. Let it rest briefly before serving.