Use a can opener to open the black beans, then rinse them thoroughly under cold water until the liquid runs clear. Transfer the beans to a large mixing bowl.
Finely chop the red onion and add it to the beans. The small dice will add a crisp, sharp bite to contrast the creamy beans.
In a small jar or bowl, combine the olive oil, freshly squeezed lime juice, ground cumin, and a pinch of salt. Shake or whisk until well blended and fragrant.
Pour the dressing over the beans and onions, then add the chopped cilantro. Gently toss everything together so the beans are evenly coated and herbs are distributed.
Taste the salad and add more salt or lime juice if needed. The flavors should be bright, with a good balance of acidity and herbaceous freshness.
Let the salad sit in the fridge for at least 30 minutes to allow the flavors to meld and develop depth.
Give the salad a gentle toss before serving. Garnish with extra cilantro if desired for a fresh, vibrant finish.