Defrost the shrimp and dry with a paper towel, chop coarsely.
Mix the chopped shrimp with the celery, bell pepper, lemon juice, and mayonnaise.
Add salt and pepper to taste. Taste and adjust the mayonnaise and seasonings, as desired.
Place half the lettuce in a bowl, add one half of the avocado, season with a pinch of salt and pepper. Scoop the shrimp salad into the cavity of the avocado and garnish with cherry tomato halves and herb(s), if using.