To cook shrimp: Fill a large pot with water and add 1 onion (quartered), 1 celery stalk (cut into pieces), 1 lemon (sliced) and 4 flat-leaf parsley sprigs. Bring to a boil over high heat, reduce the heat and simmer for 15 to 20 minutes. Add the shrimp, and a tablespoon of salt to the shrimp. As soon as the water returns to a simmer (about 3 minutes), remove from the heat. The shrimp will have just begun to curl and have turned a bright pink. Do not allow the water to boil, or the shrimp will be tough. Drain—but do not rinse the shrimp, or the flavor will go right down the drain. Reserve the broth, if desired. Allow the shrimp to cool.