Raw onion can be the first and last thing you taste in a mild bean salad. Finely cutting a small amount distributes the flavor more evenly than adding large chunks.
If you prefer a gentler result, soak the cut onion briefly in cold water, then drain and dry it well before mixing. This changes the bite but does not turn a strong onion into a different ingredient. You can also leave it out and use chives or spring onion if those suit the dish.
Taste onion with the beans and dressing together. A piece eaten alone may seem much stronger than it does in the full mouthful. Start with less than the whole onion; there is no rule that every recipe must use an entire vegetable simply because you have cut into it.