Drain and rinse the black beans thoroughly under cold water, then transfer them to your large mixing bowl.
Finely chop the cilantro leaves and tender stems, then add them to the bowl with the beans for a burst of fresh herbal aroma.
Slice the red onion as thin as possible, then soak the slices in cold water for 10 minutes to mellow their sharpness. Drain thoroughly and add to the bowl.
Squeeze the juice of one lime over the bean mixture, giving it a bright, citrusy aroma and flavor.
In the small bowl, whisk together the olive oil, cumin, salt, and pepper to make a simple dressing. Pour this over the salad and toss gently to coat all ingredients evenly.
Gently fold in halved cherry tomatoes and crumbled feta cheese, if using, for pops of color and flavor.
Let the salad sit at room temperature for 15 minutes, allowing flavors to meld and the herbs to release their aroma.
Give the salad a final gentle toss, taste, and adjust seasoning with more lime juice or salt if needed.
Serve the black bean salad in bowls or on a platter, garnished with extra cilantro or cherry tomatoes for a colorful presentation.