Cut the head of lettuce in half through the core.
Carefully hollow the lettuce half, making two bowls with walls approximately 1/2-inch thick.
Save some of the lettuce from the inside of the head.
If a bowl is too wobbly, carefully slice off a thin circle from the base, taking care not to cut through into the inside of the bowl.

Chicken
To help the chicken breasts cook quickly and evenly, slice them in half horizontally, holding the top with the flat palm of your hand to help keep the two halves to an even thickness. Rub both sides of each piece of chicken with a good pinch of the Cajun seasoning.  Pan-broil or grill the breasts over a medium-hot charcoal or gas fire for 3 to 4 minutes per side, until firm and done through to the center (if unsure, cut one open to test for doneness). Cut the chicken into 1-inch chunks and place in a large bowl with the reserved shredded lettuce. Add the blue cheese dressing and toss together quickly. Pile into the iceberg bowls and sprinkle with parsley.
Serve immediately, with lemon wedges on the side.

Shrimp (alternative)
Sprinkle with Cajun seasoning and pan-grill or grill over hot charcoal for 2 to 3 minutes per side, until pink and opaque.

Let cool to room temperature and refrigerate until chilled, about 1 hour.
If boiling the shrimp: Bring 2 quarts of water to the boil in a large pot with the crab boil and a pinch of salt.
Place a large bowl of cold water and ice near the sink. 

Add the shrimp to the boiling water, cover the pot, and return to a boil.
Boil for one minute, then drain the shrimp and plunge into the iced water to stop the cooking and quickly chill the shrimp.
Shred the reserved lettuce and place in a medium mixing bowl.
Add the cooled drained shrimp and the blue cheese dressing. 

Toss together quickly and pile into the iceberg bowls.
Sprinkle the salads with parsley and serve immediately, with lemon wedges on the side.