French onion soup has a reputation as restaurant food — the kind of thing that arrives in a little crock with a dramatic cheese pull and a $14 price tag. At home, it is one of the cheapest soups you can batch cook, because its soul is just onions, patience, and broth. When you batch cook french onion soup in the slow cooker, the caramelization that normally demands an hour of stirring happens while you are at work, and a giant pot of it costs less than a single bistro bowl. Onions are practically free in winter. A 10-pound bag at No Frills or Walmart Canada routinely sells for under…