How does Henry get the nickname, Little Onion, in the novel, The Good Lord Bird?

When Henry is first taken to John Brown's army encampment, Brown hands Henry a dusty old onion. Henry is terrified and eats it, thinking he is being offered food. However, the onion turns out to have been one of Brown's many good luck charms. As a result of this incident, Brown gives Henry all of his good luck charms, and gives Henry the nickname "Little Onion." He believes that Henry is now his human good luck charm, and believes Henry's presence brings him victory and protection in battle.