In a saucepan, bring 3 cups water, tarragon stems, lemon, 1/4 teaspoon salt, and peppercorns to a boil.
Remove from heat, add shrimp and poach
until opaque, about 2 to 3 minutes. Transfer
shrimp to a plate (discard everything else) and
refrigerate to cool.
In a bowl, whisk together
the oil, vinegar, chives, tarragon, 1/4 teaspoon salt, and
pepper to taste. Add shrimp, apple, and cucumber,
and toss to coat.
Divide salad between
four bowls. Add avocado to the mixing bowl and
toss with remaining dressing. Top each salad
with a quarter of the avocado. Serve immediately.