How to Make It
Preheat the oven to 450°. Spread 1/4 inch of rock salt in a pie plate. Arrange the potatoes on top of the salt, cover with foil and bake for about 40 minutes, until they are tender. When the potatoes are cool enough to handle, peel and pass them through a ricer into a bowl. Add the sour cream, egg, flour, salt and nutmeg to the potatoes and stir gently until incorporated.
Transfer the potato mixture to a resealable plastic bag and cut 1/2 inch off one of the corners. Line a baking sheet with parchment paper and dust with flour. Pipe 1-inch gnocchi onto the baking sheet and cover with plastic wrap.
In a small bowl, blend the butter with the garlic and parsley and season with salt and pepper.
In a small saucepan, bring the white wine to a simmer, add the snails and cook over low heat for about 5 minutes.
In a large, deep skillet, heat the olive oil. Add the oyster mushrooms and season with salt and pepper. Cover and cook over moderately high heat, stirring a few times, until starting to brown, about 3 minutes. Uncover and cook, stirring, until the mushrooms are browned and tender, about 4 minutes longer. Add the snails and 1/2 cup of the wine they were in and bring to a simmer. Stir in the garlic butter and sherry vinegar and remove from the heat.
In a large pot of boiling salted water, cook half of the gnocchi until they float to the surface. Using a slotted spoon, transfer the gnocchi to the sauce. Repeat with the remaining gnocchi, gently folding them all into the sauce. Transfer the gnocchi to bowls. Sprinkle with the Parmigiano-Reggiano cheese and watercress and serve.