Cook potatoes in boiling salted water until very tender. While the potatoes are cooking, whisk together the ingredients for the vinaigrette in a medium bowl.
Cut the cooked potatoes into quarters and put them directly into the vinaigrette while they are hot to infuse flavour.
Zest 2-3 tablespoons of grapefruit peel. Then, peel and segment the grapefruit and place in a strainer over a bowl to save juice for searing the prawns.
Thinly slice the fennel and cucumber.
Peel and cube the avocado and persimmon.
Pluck individual tarragon leaves from the stem. Set all ingredients aside.
In a hot sauté pan, sear prawns in one tablespoon of olive oil. Cook the prawns for approximately one or two minutes on each side or until cooked.
Add grapefruit zest and reserved juice to pan. Combine prawns and remaining salad ingredients together, toss and serve.