Remove spinach stems, wash, dry and reserved spinach leaves. Split leek in 4, lengthwise, and rinse under cold running water. Mince the leek and sweat in 1-ounce butter with the spinach. Cook until melted on medium heat, stirring continuously. Season with a pinch of salt and pepper, and mix well. Strain in a sieve when vegetables are cooked.

In a small saucepan, melt and clarify the remaining butter, then set aside.

Place the salt in a baking dish, open the oysters, and separate the meat from the shell by cutting the muscle that holds them. Place in a pan, and stabilize the bottom shells on the layer of rock salt. Poach oysters in their juice and place them back in their shells on a bed of the leeks and spinach mixture.

Place the yolks and white wine in a small bowl, emulsify on medium heat and whisk this sabayon until thick and the ribbon stage is reached. Do not leave over the heat too long, or you will cook the eggs.

Remove from the heat, incorporate clarified butter and season with salt, pepper and a small pinch of cayenne. Cover oysters with the sauce and bake under the grill until golden brown, about 2 minutes. Serve immediately.