Gently separate the leaves of the little gem lettuce heads and arrange on a large salad plate.
Over the lettuce leaves arrange the blanched sugar snap peas, radish slices, avocado wedges, and sliced shallot. Set aside
In a medium bowl, place the egg yolk, lemon juice, tarragon vinegar, and dijon mustard, whisk to combine.
Whisking constantly, slowly drizzle in the olive oil to create a creamy, emulsified dressing, the consistency should be thick but pourable. Taste and add salt as needed. Fold in the finely chopped tarragon.
Drizzle on top of the salad, reserving a small amount for the prawns.