If you feed a cow an onion will its milk have taste?

Yes. Cow's milk can take on the flavor of the foods that the cow eats. Cows that eat onions will produce milk that has a noticeable onion flavor. This is because the compounds that give onions their characteristic flavor are absorbed into the cow's bloodstream and then passed into the milk. The same is true for other foods that have strong flavors, such as garlic, cabbage, and broccoli.