300 g / ⅔ pound waxy potatoes, peeled
100 g / ⅓ cup plus 2 tablespoons unsalted butter
1 tablespoon coarse Dijon mustard
Fine sea salt
Ground nutmeg
500 g / 2 cups plus 2 tablespoons vegetable oil for frying

2 whole whitefish, scaled, gutted and cleaned
100 g / 3½ ounces hay
Fine sea salt
Herb oil, or olive oil

200 ml / ¾ cup plus 2 tablespoons buttermilk
50 g / 2 ounces sour cream
Zest of ½ lime
Fine sea salt
Ground pepper

60 g / ¼ cup whole milk, preferably Alpine milk

4. Remove the belly flaps off the whitefish and cut the fish into 4 fillets. Place half of the hay in a large, low casserole dish with a tight fitting lid, arrange the fish fillets on top of the hay and season to taste with salt. Drizzle with a little herb oil or olive oil, cover with the remaining hay and cover the casserole dish. Bake for 8–10 minutes or until the fish is just cooked through.