Heat large pot of water to boiling. Immerse tomatoes in water for 30 seconds. Remove with slotted spoon and let cool several minutes. Peel tomatoes, then cut crosswise in half and squeeze out seeds. Coarsely chop tomatoes and set aside.
Add green beans to boiling water and cook 5 minutes. Remove with slotted spoon, cool under running water, and drain well. Refrigerate covered until just before serving.
Add barley to boiling water and boil until tender but still firm to the bite, 40-45 minutes. Drain well.
Mix oil, vinegar, thyme, salt, and pepper in large bowl. Add tomatoes and barlsey and stir to coat. Refrigerate at least 30 minutes.
Just before serving, stir in beans, parsley, and scallions. Serve cold.