Prepare shrimp:Bring a large pan of salted water to a boil. Add shrimp, then remove from heat and let stand in water until cooked through, about 5 minutes. Drain and cool to room temperature. Cut shrimp into thirds and transfer to a large bowl with celery and scallions.
Make sauce:Whisk together all sauce ingredients, salt and pepper to taste.
Just before serving, stir sauce into shrimp mixture. Spoon into martini glasses and serve.