1. Preheat the oven to 180°C.

2. Peel the potatoes and cut into thin slices.

3. Butter 4 individual gratin dishes. Place the potato slices; season with salt, pepper, and nutmeg.

4. Grate the orange zest.

5. In a saucepan, heat the milk with the cream and zest until boiling; remove from heat and let infuse for 5 minutes.

6. Remove the zest and pour the milk over the potatoes, just to cover them.

7. Cook the gratins for 45 minutes in the oven, until the top is golden brown and the potatoes are tender.

8. Rinse the red mullet fillets; dry, salt, and pepper on both sides. Place on the gratins, drizzle with a little melted butter, and return to the oven for 7 minutes.

9. Grate the lemon zest and chop the mint leaves; mix.

10. Remove the red mullet from the oven; cover with the zest and mint mixture. Serve immediately.