In a medium saucepan, combine sugar, 2 tbsp (30 mL) water and lemon juice. Place remaining 4 tbsp (60 mL) water near stove. Bring sugar mixture to a boil over medium heat, stirring constantly; cook for 5 minutes or until mixture is dark amber in colour. Remove from heat.
Gradually and carefully add water to sugar mixture or it may spatter. Stir until sugar mixture is dissolved. In a large bowl, whisk together eggs and milk. Stir in caramel mixture. Cook over medium-low heat for about 12 minutes or until mixture coats back of spoon. Immediately strain through a cheesecloth into a 6 cup (1.5 L) bowl. Stir in vanilla, nutmeg and salt.
Cover and refrigerate until chilled. Just before serving, beat together whipping cream, liqueur, if desired, and icing sugar until soft peaks form. Pour chilled eggnog into2 quart ( 2 L) serving bowl. Fold in whipping cream with whisk. Sprinkle nutmeg on top, if desired.
Tip: For a hint of holiday spirit, add dark rum to taste. Eggnog can be prepared ahead of time. It can be kept, covered, 3 to 4 days in the refrigerator. Whisk eggnog before serving.