Fill a large pot with water, and mix in the onion, lemon, bay leaves, peppercorns, and salt. Bring to a boil. Place the lobsters in the boiling water, and cook until bright red, about 12 minutes for 1 pound, and about 5 more minutes for each additional pound.
Remove the cooked lobsters from the pot, and rinse under cold water. Chill in the refrigerator, and serve cold.
In a small bowl, mix the mayonnaise, ketchup, and brandy. Serve as a dipping sauce with the lobster.