In a large mixing bowl, stir together the yogurt, half-and-half, cucumbers, red onion, scallions, salt, and pepper. Transfer the mixture in batches to the bowl of a food processor fitted with the steel blade. Process until the cucumbers are coarsely pureed and then pour into another bowl. Continue processing the soup until all of it is pureed. Fold in the dill, cover with plastic wrap, and refrigerate for at least 2 hours, until very cold.
Just before serving, stir in the lemon juice. Serve chilled, garnished with the shrimp, lemon, and fresh dill.
2008, Barefoot Contessa Back to Basics, All Rights Reserved (quantities changed for television)