Heat the butter, sweat the flour in it, add the mustard and pour the broth and cream on top. Season with salt, pepper and sugar.
Whisk 3 egg yolks, add some of the hot sauce and whisk together. Then stir the whole mixture into the sauce, carefully heat and mix.
Heat the oven to 250 °.
Put the drained rice in a greased baking dish. Season the fish with salt, pepper and lemon juice and place on top of the rice. Pour the sauce over it and bake the fish in the preheated oven for 15-20 minutes.