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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