Fill a quart kettle 3/4 full of salted water and bring to a boil for the beans and orzo.
Chop the onion and mince the garlic.
Quarter and seed the tomatoes.
Cut the tomato quarters lengthwise into 1/4" thick slices.
In a large skillet, cook the onion and garlic in oil over moderate heat until the onion is softened.
Add in the tomatoes and cook until softened, about 2 minutes.
Remove the skillet from the heat.
Set out a bowl of ice with cold water in it.
In the salted water, blanch the beans for 1 minute.
Transfer the beans into the ice water with a slotted spoon to stop cooking.
Drain in a colander and pat dry.
Add the beans to the tomato mixture and return the kettle water to a boil.
Boil the orzo until al dente and then drain.
Add the orzo to the bean mixture with vinegar, parsley, feta and salt and pepper to taste.
Toss well to combine.