Add the shrimp and return to a boil. Immediately remove the saucepan from the heat and let the shrimp steep for 3 minutes. Drain and discard the herbs and vegetables. Peel and split the shrimp in half lengthwise; remove any veins.
Put the bell pepper, scallion, parsley, chives, tarragon, mayonnaise, and Tabasco in a medium bowl and mix together. Add the shrimp, toss, and taste to adjust the salt and Tabasco, if necessary.
Toast the cornbread for 3 minutes on each side and divide among 6 plates. Spoon the shrimp salad over the cornbread crostini. Garnish with the chervil sprigs or with more chives and parsley.
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