Boil potatoes till cooked but still firm. Drain and put through potato ricer. Add egg, salt and flour and knead until combined, on a well-floured bench. Divide dough into balls, rolling each ball into sausages and cut into bite sized pieces. Roll each piece over the back of a fork, leaving a slight indentation. Cook gnocchi in batches in a pot of boiling salter water until they float to the surface of the pot.
Prepare sauce. Fry onion in olive oil until lightly brown then lightly toast the garlic. Add wine and reduce by half then blend until smooth. Add the stock and cream and reduce till the sauce coats the back of a spoon. Add gnocchi and the grated black truffle then add the spinach until wilted.
Serve with sliced black truffle. Add salt and pepper to taste.