To make the sauce, put the onions, cucumber, and olives into a sieve, sprinkle with a little salt, and drain for about 10 minutes. In a small bowl, combine the sour cream with the vegetables and season with salt and pepper. Set the bowl in a bowl of ice and chill in the refrigerator for half an hour so that the sour cream can set up again. (Make a sandwich from the unused cucumber half, since it won’t keep.)
To make the calas, put the rice in a large mixing bowl. Purée the scallions in a food processor until almost liquid and fold into the rice. Combine the flour, salt, and baking powder and stir into the rice. Fold in the eggs, followed by the seafood. Let the batter rest in the refrigerator for half an hour.
Using a small ice-cream-style scoop, form the batter into 1-inch balls. Fill a straight-sided saucepan with enough oil to float the calas and heat it to about 360°. (If you don’t have a thermometer, you can test the temperature with fair accuracy by dropping a few specks of the batter into it. If the oil is ready, the batter will sizzle, float, and brown quickly.) Place as many calas into the oil as you can without crowding them. As they cook, they will float and brown. Usually they will turn themselves over as they cook; if not, do this with tongs. Fry for 4–5 minutes. Break one open to make sure they are done through. Serve hot with the sauce.