Fill a 4-quart kettle three fourths full with salted water and bring to a boil for beans and orzo.
Chop onion and mince garlic. Quarter and seed tomatoes. Cut quarters lengthwise into 1/4-inch-thick slices. Trim beans and cut into 1-inch pieces. In a large skillet cook onion and garlic in oil over moderate heat until onion is softened. Add tomatoes and cook, stirring, until tomatoes are softened, about 2 minutes. Remove skillet from heat.
Have ready a bowl of ice and cold water. In boiling salted water blanch beans 1 minute. With a slotted spoon transfer beans to ice water to stop cooking. Drain beans well in a colander and pat dry. Add beans to tomato mixture and return water in kettle to a boil. Boil orzo until al dente and drain in colander. Add orzo to bean mixture with vinegar, parsley, feta, and salt and pepper to taste, tossing to combine well.
