Slice the avocado in the middle, remove the stone and spoon the pulp onto a large plate. Drizzle the avocado with 1 tablespoon of lemon juice to prevent the meat from browning and mash it with a fork. Cut the onion into very fine cubes and use a fork to mix with the cream cheese into the avocado mousse. Season with salt and pepper and add the remaining lemon juice to taste. Cut the hard-boiled eggs into small pieces and carefully stir into the spread.