Wash shrimp and put them in a large kettle.
Add onion, lemon, bay leaf, celery leaves, parsley, peppercorns and salt.
Cover with boiling water and simmer about 5 minutes or until shrimp are tender.
Cool, shell, clean and chill.
Carry to picnic in a cooler and serve with Mustard Sauce.
Note: Fairly large shrimp come about 22 per lb. Extra-large come about 16 per lb.