Trim ends of beans and cut into bite-sized pieces, washing if needed.
Bring a medium-sized pot of water to a boil, add a generous amount of salt, then add beans and cook for 3-4 minutes, or until barely tender-crisp. (Taste a bean to be sure.) While beans cook, cut cherry tomatoes in half and slice green onions in thin diagonal slices.
When beans are tender-crisp, drain into a colander placed in the sink, then rinse beans with cold water until they're cool. Drain beans again, then put them between a double layer of paper towels and blot dry. (Don't skip this step or the salad will be watery.)
Combine beans, tomatoes, and green onions in a salad bowl, then toss with the tangy dressing, stirring until the vegetables are well-coated with dressing. Add crumbled Feta and stir just to combine. Serve immediately.
I didn't think this would keep well in the refrigerator because chilling tomatoes takes away their flavor, but it was actually still very good after a night in the fridge. I wouldn't purposely make it ahead, but if you have leftovers, go ahead and save them and you might be surprised.