Bring a large pot of salted water to a boil over high heat.
Add green beans and cook until crisp-tender, 3-4 minutes. They should still have a slight snap.
Drain green beans immediately and transfer to a large bowl of ice water to stop cooking. This keeps them bright green and crisp.
Remove from ice bath and pat completely dry with paper towels.
Add dried green beans to a large glass or non-reactive bowl. Add tomatoes, basil, parsley, olive oil, and sherry vinegar. Toss to combine. Season with salt and pepper.
Gently fold in crumbled feta cheese.
Serve immediately warm, or chill and serve cold.