Make dressing by whisking together all ingredients, then refrigerate.
In large pot of water, combine bay leaf, 2 celery ribs, parsley and peppercorns and bring to a boil.
Reduce heat to medium and add shrimp. Cook until they float and are opaque, 45-60 seconds. Strain and place shrimp in bowl of ice to chill.
When cool, cut shrimp into 1-inch pieces.
Finely dice 1 celery rib and place in large bowl with shrimp, shallots and tomatoes.
Using a melon baller or small spoon carve small balls from each avocado to make a boat.
In small bowl, combine pieces of avocado with lime juice to prevent browning and toss gently. Strain avocado pieces from juice and add to shrimp mixture. Coat the avocado shells with remaining lime juice.
Fold the cilantro dressing gently into the avocado/shrimp mixture.
Fill the reserved avocado boats with shrimp mixture and garnish with cilantro leaves. Serve immediately.