For the shrimp: In a large saucepan, combine 2 1/2 quarts water with the salt, red pepper flakes, black peppercorns, garlic, bay leaves, lime and parsley. Stir to combine and bring to a simmer. Remove from the heat and allow the liquid to cool for a few minutes (dropping shrimp in hot, but not boiling, water yields more tender shrimp). Prepare and clean the shrimp by running a small, sharp knife down the length of the shrimps and removing the dark vein. Add the shrimp to the poaching liquid and allow them to sit and steep in the liquid, off the heat. (Cooking the shrimp in the shell yields more flavor.) Leave for about 5 minutes until they are cooked through and completely pink. Use a slotted spoon, or spider, and remove the shrimp and drain well. Refrigerate until cool, and then peel and discard the shells before serving with the salad.
For the dressing: Combine the mayonnaise, sour cream, basil, parsley, green onions, lemon juice, anchovy paste and garlic in a food processor and puree. Thin with water as required, using only 2 tablespoons at a time. Taste and season with salt and pepper.
For the salad: Wash the lettuces and roughly chop the romaine hearts. Tear the Bibb lettuce leaves into bite-size pieces. Cut the cucumber lengthwise down the middle, and then slice in 1/4-inch slices on the bias. Toss the salad ingredients together with the shaved red onion, pepperoncinis, green onions, bell peppers and cucumber. Top with the shrimp. Dress lightly with the dressing.