Butter one side of the sliced bread or cut the side of 6-inch lengths of halved and inverted French bread.
Build the sandwiches with the buttered sides out; put Camembert, onions and shredded Comté or Gruyère cheese on top of one slice of bread, then cover with another slice. Place the sandwich in skillet over medium heat, cover with foil and grill for 7-8 minutes. Turn occasionally and press as the sandwich turns golden brown. Serve with cornichons and Dijon mustard.