Preheat the oven to 180°C. Grease the holes in a muffin tin.
Finely chop the onion, then coarsely grate the potatoes.
Melt butter in a frying pan, then sauté the onion over low temperature for 5 minutes, or until soft (but not brown).
Mix the grated potatoes with the onion and the egg. Season with salt and pepper, then gently combine.
Add a spoonful of the mixture to each muffin hole and, using the base of the spoon or your fingers, gently press the mixture down and to the sides to create nest shapes.
Put the tin in the oven and bake for about 15 minutes until it’s golden and crispy.
Take the tin out of the oven, rest for a couple of minutes, then carefully remove the nests to a plate.
Fill the nests and serve warm or at room temperature.