Whisk together the white balsamic vinegar, olive oil, black pepper, and sea salt.
Toss the beans and vinaigrette in the medium bowl, then add the parsley, oregano, lemon thyme, onion slices, and feta cheese. Serve immediately or chill an hour or two before serving.
Variations
Use dry beans: Rinse 1 cup dry beans until clean. Cover with water and soak overnight. Drain, then add fresh water to cover. Cook over medium heat until the beans are tender, 1 to 2 hours. Drain and cool.
Add fresh halved cherry or grape tomatoes when in season.