Place the finely chopped onion into a non-metallic bowl. Squeeze over the lemon juice, then season with the salt and black pepper. Stir thoroughly so the onion is evenly coated, allowing the acidity to soften its sharpness.
Set the bowl aside for about 5 minutes, which helps mellow the onion while you prepare the remaining vegetables. Add the chopped tomatoes, cucumber, red bell pepper, olive oil, parsley, and chives to the bowl.
Mix everything gently but well until the salad is evenly combined and lightly dressed. Taste and adjust the seasoning as needed, adding extra salt, pepper, or lemon juice to achieve a bright, balanced finish.