Feta and white beans provide plenty of protein in this main-course salad. If you don't have white-wine vinegar, you can substitute red-wine or balsamic vinegar.
In a medium saucepan of boiling salted water, cook green beans until crisp-tender, 4 to 6 minutes. Drain green beans, and rinse with cold water to stop the cooking.
Using a sharp knife, slice off both ends of each orange. Cut off peel, following the curve of the fruit. Halve fruit from top to bottom, and thinly slice each half crosswise.
In a bowl, whisk together oil and vinegar; season with salt and pepper. Add feta, oranges, lettuce, onion, beans, and green beans. Toss to combine.
Make the dressing in the same bowl you'll use to toss and serve the salad. There'll be one less dish to clean up later.