Four 1/2-pound baking potatoes, pierced all over with a fork
2 large eggs, beaten
1 1/2 t kosher salt
3/4 c plus 2 tablespoons all-purpose flour, plus more for dusting
4 T unsalted butter, softened
2 1/2 T finely chopped mixed herbs, such as parsley, chives, tarragon, thyme and chervil
1/4 c extra-virgin olive oil
1 small fennel bulb, cored and thinly sliced, fronds chopped and reserved for garnish
2 shallots, thinly sliced
1 lb mixed mushrooms, such as oyster, cremini and stemmed shiitake, cut into 1-inch pieces
1/3 c dry white wine
1/2 c chicken stock or low-sodium broth
Finely chopped skinned toasted hazelnuts, for serving
Preheat the oven to 400°. Bake the potatoes directly on the oven rack for about 1 hour or until tender. Transfer to a work surface and let cool slightly. Halve the potatoes lengthwise. Scoop the flesh into a ricer and rice the potatoes. Spread the riced potatoes on a work surface and let cool completely. Discard the skins.
On the work surface, gather the riced potatoes into a loose mound and make a well in the center. Add the eggs and salt to the well. Sift the flour on top and, using a bench scraper, mix everything together until a dough forms. Knead gently until smooth; the dough will be slightly sticky.
Line a baking sheet with wax paper and dust with flour. On a floured work surface, cut the dough into 8 pieces and roll each piece into a 3/4-inch-thick rope. Cut each rope into 1-inch lengths. Roll the gnocchi against the tines of a fork or a gnocchi paddle to make ridges; transfer to the prepared baking sheet.
In a small bowl, blend the butter with the mixed herbs. Season the herb butter with salt and pepper.
In a very large skillet, heat the olive oil. Add the sliced fennel and shallots and cook over moderately high heat, stirring, until just starting to soften,3 to 5 minutes. Add the mushrooms and season with salt and pepper. Cook over high heat, stirring occasionally, until the mushrooms are browned and tender, about 10 minutes. Add the wine and cook, stirring, until absorbed, about 2 minutes. Add the chicken stock and cook over moderate heat, stirring, until the mushrooms are coated in a light sauce, 3 to5 minutes. Remove from the heat and stir in the herb butter. Season the ragù with salt and pepper; keep warm.
In a large saucepan of salted simmering water, working in2 batches, cook the gnocchi until they rise to the surface, then simmer until cooked through, 1 to 2 minutes longer. ºUsing a slotted spoon, transfer to shallow bowls or a platter.
Spoon the ragù over the gnocchi and top with chopped hazelnuts, shredded pecorino and fennel fronds. Serve right away.
The uncooked gnocchi can be frozen on a baking sheet, then transferred to a sturdy plastic bag and frozen for up to 1 month. Boil from frozen.