How to Make It
Preheat the oven to 450°. Pierce the potatoes all over with a fork and bake for 1 hour, or until tender. Let cool slightly, then halve lengthwise and scoop out the flesh. Pass the potatoes through a ricer or food mill into a large bowl. Measure out 2 cups and let cool slightly. Save any remaining potatoes for another use.
In a medium bowl, using a handheld mixer, beat the potatoes at low speed with the egg, nutmeg and 1/2 teaspoon of salt just until combined. Mix in the flour until a soft dough forms.
Line a baking sheet with wax paper and dust it with flour. On a lightly floured surface, gently knead the dough for 1 minute, until smooth. Quarter the dough and roll each quarter into a 1/2-inch-thick rope; cut the ropes into 1-inch pieces and transfer to the baking sheet. Hold a fork with the tines pointing slightly downward and gently roll each piece of dough against the tines to make slight grooves.
Bring a large saucepan of salted water to a boil. Add the gnocchi and cover. When the water returns to a boil, uncover, stir gently and boil for about 1 minute, or until tender. Using a slotted spoon, lift the gnocchi into a strainer to drain.
Melt the butter in a large, deep skillet. Add the sage and parsley and cook over moderately high heat until the sage is crisp and the butter starts to brown, about 3 minutes. Add the gnocchi and lemon zest and toss well. Add the peas, tomatoes and a generous pinch of salt and pepper and cook, tossing, until heated through, about 2 minutes. Add the Parmesan and toss to combine. Transfer the gnocchi to bowls and serve.