Health-conscious cook routinely toss the yolks and stick with the whites when preparing eggs. Good idea? No necessarily. Eggs are packed with nutrients, and most of them - vitamins A and E, folate, calcium, iron and lutien - are in the yolks. Sure yolks also pack dietary cholestrol, but many scientists now think saturated fat is the biggest culprit in raising blood cholestrol. A good srategy is to hedge your bets by using some, but not all of the yolks. For instance, preparing an omlette using one whole egg and two egg whites. This Mexican frittata calls for a reduced number of egg yolks, yet still has great flavour and texture. A wedge of this frittata, served with a salad and some crusty bread, is perfect for a light supper or brunch. It reheats well, so leftovers make a convenient meal the following day.