"All you need is one pot to make a winning soup. It's full of thyme and topped with grated cheese, and you'll want seconds (and thirds)...."
olive oil
pounds yellow onions (about 5 large onions), thinly sliced
Kosher salt
all-purpose flour
cognac or dry white wine
32-oz containers low-sodium beef broth
bay leaves
large sprigs thyme, plus more for sprinkling
sherry vinegar
thin slices country bread
Gruyère cheese, finely grated