If using frozen mashed potato, defrost it thoroughly in the microwave.
Combine the mashed potato, flour and egg in a large mixing bowl, along with a good pinch of salt and pepper. Using clean hands, mix well, and knead lightly to form a dough. If the dough is too sticky, just add a tiny bit more flour.
Roll out the dough (or just press it out with your hands) until it's around 1.5 cm thick, and cut into 1.5cm strips. Roll each strip a little on the worktop to form a sausage, and cut into pieces - mine measured around 3cm long. Use a fork to add grooves if you wish (I didn't bother).
When all of the dough has been used to form the gnocchi, bring a large pan of water to the boil. Add some of the gnocchi - you'll need to cook it in 2 or 3 batches to avoid overcrowding the pan. When the gnocchi floats to the surface (which should take less than a minute), give it another 20 seconds or so, then remove it from the water using a slotted spoon. Repeat until all of the gnocchi have been cooked.
The gnocchi are now edible, but I think they're much nicer if you fry them a little first. Heat some oil and butter in a frying pan, and add some of the gnocchi (again, cook it in a couple of batches so you don't overcrowd the pan). Cook over a medium heat for a couple of minutes each side, until golden brown and crispy.
Serve the gnocchi with your choice of sauce. I used a creamy kale pesto - if you'd like to do the same, just add the garlic, kale, coriander, and pine nuts to a mini food processor and blitz until a paste is formed. Add the extra virgin olive oil and cream cheese, along with a good pinch of salt and pepper, and blitz again. Serve the gnocchi with the pesto and a few cherry tomatoes.