In a large pot, bring at least 1 gallon plus 1 quart (20c) water to a boil over high heat. Plunge in the lobsters, cover the pot, reduce the heat, and simmer for 6 to 7 minutes, depending on the size of the lobsters. Remove the lobsters with tongs and set them on rimmed baking sheets to cool. When they are cooled to room temperature, remove the meat from the claws and tails and rip it into large chunks (I do this with my hands; I don't chop it).
Sprinkle the lemon juice, celery salt, and white pepper over the lobster. Fold in just enough mayonnaise to coat the meat and hold it together-not a lot. Put the lettuce leaf in the bun and top with the lobster mixture. Sprinkle parsley on top to garnish.