Put the potatoes, egg and flour in a bowl and mix until well combined.
Dust your counter with a little flour. Pick up the potato dough and form it into a ball, rolling it on the counter. It should hold together without being too sticky – you can add a little extra flour if you need to.
Cut the ball into four sections, and then roll each section into a rope – you want it to be about one inch thick or so.
Use a sharp knife to cut the ropes into 1 inch long gnocchi. If you want to get extra fancy, you can gently press the tine of a fork into each one to get those classic gnocchi lines – they help hold the sauce!
Bring a pot of water to a boil and salt it heavily. Carefully drop in the gnocchi and cook them until they are done, about 5 minutes – they will float to the surface when they are finished cooking. Scoop them out with a slotted spoon.
Divide them among plates and serve with tomato sauce, butter, pesto or whatever sauce your heart desires!
Notes
Mashed potatoes: Plain leftover mashed potatoes work best. Potatoes made with butter and a little milk or cream are fine, but avoid very loose or heavily loaded mashed potatoes, since they may require more flour.
Egg: One egg helps bind the potatoes and flour together so the gnocchi hold their shape while cooking.
Flour: Regular all-purpose flour is perfect. Start with the amount called for, then add a little more only if the dough is too sticky to handle—too much flour can make the gnocchi heavy instead of tender. Keep extra flour handy for dusting your work surface.