How to Make It
In a medium saucepan, cover the potatoes with water and bring to a boil. Cook over moderately high heat until tender, about 25 minutes. Drain and let cool slightly, then peel the potatoes. Meanwhile, in a medium saucepan, boil the stock until reduced to 2/3 cup, about 15 minutes. Transfer to a bowl and stir in the cream. Wipe out the saucepan.
In a large bowl, combine the flour with 1 teaspoon of salt and half of the olives. Pass the potatoes through a ricer set over the bowl with the flour. Add the butter and stir until a stiff dough forms. Turn the dough out onto a lightly floured work surface and knead it gently until smooth, 2 or 3 minutes. Quarter the dough. On an unfloured work surface, roll each piece of dough into a 1/2-inch-thick rope. Cut the ropes into 1-inch lengths and transfer the gnocchi to a lightly floured baking sheet.
Bring a large pot of salted water to a boil. Heat the olive oil in the medium saucepan. Add the onion and garlic, cover and cook over moderately low heat, stirring occasionally, until softened, about 8 minutes. Add the cream mixture and boil until reduced slightly, about 5 minutes. Add the remaining olives and 2 tablespoons of the parsley and season with salt and pepper. Cover and keep warm.
Boil the gnocchi over moderately high heat, stirring very gently, until they begin to rise to the surface. Cook the gnocchi for 1 minute longer, until they are just tender. Gently drain the gnocchi and return them to the pot. Add the green olive sauce and 1/4 cup of the Parmesan cheese, season with pepper and stir gently over moderate heat for 1 minute. Transfer the gnocchi to a serving platter, sprinkle with the remaining 1 tablespoon of parsley and 2 tablespoons of Parmesan cheese and serve.
The boiled gnocchi can be tossed with brown butter, fresh sage leaves and Parmesan cheese instead of the creamy olive sauce.