COMBINE THE LIME JUICE, oil, peppercorns, salt and pepper together in a glass or ceramic bowl. Stir in the scallops, cover and refrigerate for at least 4 hours while they marinate. They should become opaque in this time. Mash the avocado until almost smooth, then add it along with the chives or scallions to the marinating scallops (do not drain them) and mix well.
Set aside for at least 1/2 hour, refrigerated. About half an hour before serving the scallops, remove the stems from the mushrooms and wipe them to remove any dirt. Combine the vegetable oil, lemon juice, garlic, salt and pepper in a small bowl, and brush the insides of the mushrooms liberally with the mixture. Just before serving, drain the caps and fill with the scallop mixture.
Garnish with additional chives, if desired.