Taste of onions on my breasts

Body odor, including nipple smell (sweating glands that are responsible for body odor are located around the nipples, too) is an individual characteristic, influenced by diet, gender, genetics, health and medication.

The smell is caused by bacteria that interact with the secretions of the apocrine sweat glands, which are located in the armpits, on the nipples and in the groin area. The bacteria produce thiol compounds which are from the same family of sulfurous compounds that give onions their smell.

You can't stop sweating and you can't really change your chemistry. Kill the bacteria and kill the smell and taste.