Onion seeds and nigella seeds are often confused due to their similar appearance, but they are distinct in flavor and origin. Onion seeds, derived from onion plants, have a mild, onion-like taste, while nigella seeds, also known as kalonji, are sourced from the Nigella sativa plant and boast a peppery, slightly bitter flavor. Understanding these differences can enhance your culinary creations and ensure you’re using the right ingredient for your dishes.