Preheat the oven to 400°. Melt the butter in an ovenproof saucepan. Add the onion and cook over high heat, stirring, until translucent, about 2 minutes. Add the rice and stir until the grains are coated. Add the fish stock, parsley and a large pinch of salt and bring to a boil. Stir the rice, cover and bake for 18 minutes. Remove from the oven and let stand, covered, for 10 minutes. Stir the rice and season with salt.