GUACAMOLE A traditional south-of-the-border cocktail dip, to serve with sturdy potato chips or crisp crackers. 2 peeled ripe avocados 1 small white onion, grated 1 tsp salt 1/2 tsp black pepper juice of half a lemon   Mash the avocados through a sieve, add other ingredients, mix very smoothly and serve in a bowl lined with a few lettuce leaves. Dust the top with paprika. NOTES: Avocados quickly blacken with air; guacamole should never be made more than half an hour in advance—in which case, cover the bowl with Saran wrap and keep in the refrigerator until the final moment. Guacamole variations: add 2 crumbled crisp-cooked cold bacon slices—or 2 tablespoons of ground nuts (almonds,cashews, peanuts)—or a tablespoon of chili sauce—or a tablespoon of grated raw carrot—or a tablespoon of sherry and 6 minced black olives. . . .