Cook the rice as usual, drain and place in a large, flat, greased casserole dish. Preheat the oven to 250 degrees.
Melt the butter, sweat the flour and mustard in it, pour in the broth and cream, season with salt, pepper and sugar. Whisk the egg yolks and fold them into the sauce; don`t let it boil anymore.
Season the fish with salt, pepper and lemon juice and place on the rice. Pour the sauce over it. Bake in the preheated oven for 8 minutes. Serve sprinkled with chopped dill.