Assemble
Preheat oven to 350°F (175°C).
In a large bowl or directly in the casserole dish, combine the rice mixture and the béchamel sauce until evenly coated.
Pour into a greased 2-quart casserole dish.
Add the Topping
Mix panko with melted butter, Parmesan (if using), and dried sage or marjoram.
Sprinkle evenly over the top.
Bake
Cover with foil and bake for 30 minutes.
Remove foil and bake another 15–20 minutes, until topping is golden and filling is bubbling.