Put the mashed potatoes in a medium bowl. Add the boiling water and stir well. Let cool down. Add egg, salt and pepper. Add enough flour to make a firm dough. Put the potato mixture on a floured work surface and knead lightly. Halve the dough. Shape each half into a long, thin roll. Cut into bite-sized pieces with a floured knife. Put the gnocchi in small portions in boiling water. They are ready when they float on the surface of the water. Take out of the water with a slotted spoon and serve with a sauce to your taste. The gnocchi can be frozen very easily. To do this, lay out the gnocchi on a baking sheet, let it freeze and only then fill it into freezer bags.