Cut avocados lengthwise in half and carefully remove the pit. Trim a little from the bottom so they will sit up straight, and then set a half avocado on each of 4 plates. Brush avocadoes with lemon juice. Combine salad ingredients in a bowl, then divide and pack in and on the avocadoes. If desired, garnish tops with some finely chopped red pepper and green onion.
Note: Langoustines, which look and taste like mini lobster tails, are sold in the seafood department.