Step 1Blanch the sugar snap peas in boiling salted water for 30 seconds. Remove them from the boiling water with a slotted spoon and immediately plunge them into a bowl of ice water to stop the cooking. Drain and pat dry. Bring the water back to a boil for the shrimp.
Step 2Cook the shrimp in the rapidly boiling water just until firm, about 3 to 4 minutes. Drain and chill in ice water to stop cooking. Pat dry and cut in half. Place in a large bowl.
Step 3In a small bowl, whip together the mayonnaise, shallots, chives and lemon juice. Beat in a splash of vinegar, then taste and add more if necessary.
Step 4Spoon two-thirds of the mayonnaise mixture over the shrimp and peas and stir together. The mayonnaise should lightly and uniformly coat both. If necessary, stir in a little more. Season to taste with salt and pepper. Chill before serving.