For the shrimp, fill a 6- to 8-quart pot with water and add onions, wine, peppercorns, lemons, thyme and salt. Bring pot to a boil and add shrimp until cooked through, about 3 minutes. Using a slotted spoon, remove shrimp from liquid and cool.
For the sauce, in a blender or food processor, add scallions, parsley, tarragon, fronds, aioli and lemon juice. Purée until smooth. Remove from blender, place in a medium bowl and fold in sour cream. Season to taste and serve.