It wasn't made with Gruyere cheese. The broth was ladled into a bowl
(a French Onion Soup bowl). A Holland Rusk (they are toasted rounds
that fit perfectly on an onion soup bowl) was placed on top of the
bowl. Then parmesan cheese (the cheap grated stuff) was mixed with
some onion soup broth to make a paste. This was heaped on top of the
rusk. The bowl with cheese and rusk was placed under a broiler until
the whole thing was golden brown. It worked well. Very tasty.