These light, tender gnocchi are a great use for leftover mashed potatoes.
In a medium bowl, combine potatoes, egg yolk, lemon zest, olive oil, flour, and 3/4 teaspoon salt. Using a wooden spoon, stir until incorporated. Divide dough into fourths. Line a baking sheet with parchment paper.
On a lightly floured work surface, roll 1/4 of the dough into a 3/4-inch-diameter log. Cut the log into 3/4-inch-long pieces. Pick up one piece of dough, and press onto the back of a fork with your thumb. Still using your thumb, roll dough off fork, forming a "C" shape; the indentations from the tines should be visible. Place the gnocchi on the prepared baking sheet. Repeat process with the remaining pieces of dough, arranging finished gnocchi in a single layer. Repeat with the remaining 3 portions of dough.
Fill a large saucepan with water, and add 2 teaspoons salt; bring to a boil. Have ready a colander set over a bowl. Add 1/4 of the gnocchi, and cook until they float to the top, 1 to 2 minutes. Using a slotted spoon, transfer cooked gnocchi to the colander. Cook the remaining gnocchi.
Place a large saute pan over medium-high heat. Add butter. When butter begins to brown, about 2 minutes, add pepper and the remaining 1/4 teaspoon salt. Increase heat to high. Add the gnocchi and sage. Cook, stirring occasionally, until the sage has wilted and the gnocchi have heated through, 1 1/2 minutes. Serve immediately, with a sprinkle of Parmesan cheese.