Slice each shrimp in half lengthwise.

Whisk the lemon juice, vinegar, seafood boil spice, celery seed, mayonnaise, and sour cream together in a large bowl. Add the shrimp, celery, bell pepper, and onion (if desired) and toss to coat. Cover and refrigerate until ready to serve.

Cut the avocados in half; remove pits. Use half an avocado for each appetizer serving, filling with shrimp.

Tip: Lydell "Delly" Easton of Rockville, South Carolina, suggests bruising each shrimp between your thumb and finger while still warm to break up the meat, allowing it to absorb flavors more easily.