2   (15-ounce) cans black beans, drained and rinsed
1   small red onion, finely diced

Optional Ingredients:
1 tablespoon olive oil
2 tablespoon red wine vinegar
1   garlic clove, minced
1   (4-ounce) can chopped green chiles with juice
1   small green bell pepper, seeds and veins removed, and minced
1   small onion, finely chopped
3   large ripe tomatoes, diced or 1 (14-ounce) can diced tomatoes with their juice
1   large avocado, peeled and diced
salt and freshly ground black pepper
1   mango, peeled and diced
1/2 cup pineapple chunks
2   limes, juiced