Preheat the oven to 350°F (175°C).
Place the cooked, cubed (or shredded) chicken in a 9x13-inch casserole dish; if using, spread the cooked rice in a layer on the bottom first before adding the chicken.
In a bowl, stir together the condensed cream of chicken soup and sour cream until smooth, then pour the mixture evenly over the chicken (and rice, if used).
In a separate small bowl, mix the crushed Ritz crackers with the melted butter until the crumbs are evenly coated.
Sprinkle the buttered cracker mixture evenly over the top of the chicken and sauce, then sprinkle poppy seeds over the cracker topping if using.
Bake in the preheated oven for 20–30 minutes, until the topping is golden brown and the filling is bubbly.
Remove from the oven and serve plain or over rice; optional steamed broccoli can be served alongside.
