Wash off the orange, dry it off. Rub some of the peel finely. Halve the orange. Squeeze out the juice. Peel the onion, dice it finely and mix it with the juice, zest and salt in a wide bowl.
Wash the fillet, dry it and add it to the bowl. Cover and marinate for 30 minutes. Preheat the oven to 200 degrees. Put the fillets in a baking dish, season with a little nutmeg and pepper. Cook in the oven for a good 10 minutes, drizzling the marinade over and over again. Possibly Garnish with orange slices or strips.