Cook the rice in salted water according to the instructions on the packet. And preheat the oven to 180 ° C (convection).
Cut the onion into small pieces and sauté with about half of the butter and add the spinach. Season to taste with salt, pepper and a little nutmeg. Drizzle the fish with lemon juice, season with salt and pepper.
Melt the remaining butter in a small saucepan and add the flour. Mix well, then deglaze with the milk and stock and add the mustard and honey. Season to taste with salt, pepper and dill and reduce a little.
Then first layer the cooked rice, then the spinach in a baking dish. Place the seasoned fish on top and pour the sauce over it.
Put the baking dish in the preheated oven and bake for about 15 minutes. Serve hot.