How to Make It
In a saucepan, cover the potatoes with 1 inch of water and cook over moderate heat until tender, 15 minutes; drain and pass through a food mill or ricer into a bowl. Add the egg yolks, 1 1/3 cups of flour, salt and nutmeg; gently mix until the dough starts to come together. Gently knead on a floured work surface, dusting with flour if sticky. Divide the dough into 4 equal pieces.
Roll out one piece of dough into a 3/4-inch-thick rope; keep the remaining dough covered with a damp kitchen towel. Cut the rope into 1-inch pieces and lightly dust with flour. Roll each piece against the tines of a fork or a gnocchi paddle to make ridges. Transfer the gnocchi to a lightly floured baking sheet. Repeat with the remaining dough.
Working in 2 batches, cook the gnocchi in a large pot of salted boiling water until they rise to the surface, about 2 minutes. Then simmer until just cooked through, 1 to 2 minutes. Transfer to an ice bath and let cool. Drain well and let dry on paper towels.
In a saucepan, bring the stock to a simmer. Add the crabs, cover partially and simmer over high heat until the liquid is reduced to 2 cups, 15 minutes. Strain into a bowl; discard the crabs.
In the same saucepan, heat the olive oil. Add the onion, fennel, garlic and crushed red pepper and cook over moderate heat, stirring, until tender, 5 minutes. Add the reduced crab stock, cream, vermouth, thyme, tarragon and bay leaf and bring to a boil. Simmer over moderately high heat until reduced to 2 cups, 15 minutes. Discard the tarragon and bay leaf.
In a skillet, heat half of the pan sauce. Add half of the gnocchi and cook over moderate heat until hot, 2 minutes. Add half of the crab and 1 1/2 tablespoons of the butter; cook until warmed through and the sauce is slightly thickened, 2 minutes. Season with salt and transfer to a platter. Repeat with the remaining pan sauce, gnocchi, crab and butter. Garnish with the cheese and serve at once.