Onions make a luscious sauce for pasta and this combination is delicious. Be patient and wait for the onions to cook down until they are almost creamy in consistency.
Trim the tough green tops of the leeks. Cut off the root and slice in half crosswise. Cut into very thin long strips. Place in a bowl with cold water and swish them around to loosen any dirt
Put the olive oil and shallots in a 12-inch skillet and place over medium heat