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.
Notes:
Keeps for three to four days in the refrigerator.
Variations:
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.