1 tablespoon olive oil
1 tablespoon butter
1 large yellow or Spanish onion, halved and thinly sliced
1 large red onion, halved and thinly sliced
1 tablespoon sugar
1 teaspoon dried thyme
2 bay leaves
1/2 teaspoon salt
1/4 teaspoon ground black pepper
1/2 cup port wine
6 cups reduced-sodium beef broth
1 (2-inch) piece Parmesan rind
Reserved bread from bread bowls or 4 slices sourdough baguette, about 1-inch thick and lightly toasted
8 slices Gruyere cheese