For the boiled shrimp: Add the shrimp shells, seafood seasoning, parsley, peppercorns, celery, lemon and onion to a large stock pot and fill three-quarters of the way with water. Bring to a boil and cook until the vegetables soften slightly, 5 to 10 minutes (see Cook's Note).
Prepare an ice bath in a large bowl. Add the shrimp to the stock pot and boil, stirring constantly, until pink and opaque, 1 to 2 minutes. Remove the shrimp and add to the ice bath until completely cooled.
For the salad rolls: In a large bowl, combine the mayonnaise, mustard, parsley, seafood seasoning, pimentos, celery and onion. Fold in the shrimp and crab meat. Cover and chill for at least 30 minutes and up to 2 days.
When ready to serve, fill the buns with the seafood mixture and serve with a lemon wedge and some clarified butter.
If you have more time, boil the stock for up to an hour for a more intense flavor! This is also great for freezing and using another time. The stock will keep for up to 6 weeks in the freezer.