Rinse the black beans thoroughly under cold water, then drain and place them in a large mixing bowl. If you prefer softer beans, gently warm them in a pan over low heat for about 5 minutes, then set aside to cool slightly.
Finely dice the red onion and, if you find the raw sharpness too strong, soak the diced onion in cold water for 10 minutes. Drain and gently pat dry with a paper towel.
Juice the limes using a citrus juicer until you have about ¼ cup of fresh lime juice. Chop the cilantro roughly, removing thick stems for a bright, herbal aroma.
Add the diced onion and chopped cilantro to the bowl with the beans. Drizzle in the olive oil and toss gently with a wooden spoon to coat everything evenly.
Pour in the lime juice and sprinkle the chili flakes over the mixture. Season with a pinch of salt if desired, then toss again to distribute the flavors thoroughly.
Let the salad sit at room temperature for at least 10 minutes to allow flavors to meld, or cover and refrigerate for up to 2 hours for a more pronounced flavor.
Before serving, give the salad a gentle toss, taste, and adjust seasoning with more lime juice or chili flakes if needed. Serve in bowls garnished with extra cilantro if desired.
