Combine the beans, onion, oil, vinegar, garlic, salt, and pepper in a large bowl; mix well. (Can be prepared in advance up to this point and refrigerated for up to 24 hours.)
At serving time, sprinkle with feta cheese and serve immediately.
Keeps for three to four days in the refrigerator.
Tex-Mex Bean Salad: Omit feta cheese. Add 2 cups drained corn kernels, 2 chopped red peppers, and 1/2 cup minced fresh cilantro or parsley.