There is some debate as to whether onions actually have an effect on your gallbladder. Some say that onions can help to dissolve gallstones, while others claim that they can actually cause them. However, there is no concrete evidence either way.
Onions are a high-fiber food, which means they can help to bulk up stools and make them easier to pass. This could potentially help to prevent gallstones from forming in the first place. Additionally, onions contain sulfur compounds which may help to break down existing stones.
However, onions are also high in fat and cholesterol, both of which can contribute to the formation of gallstones. So, if you already have a history of gallstones or other digestive issues, it's best to avoid eating too many onions. Talk to your doctor if you're unsure whether onions are right for you.