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Melt butter in a large saucepan over low heat. Add onions and cover. Cook for about 30 minutes, stirring occasionally.
Add the beef broth and the Worcestershire sauce. Heat the broth mixture until it boils.
Reduce heat to low. All to simmer, uncovered, for about 30 minutes. The broth will reduce.
Pour onion and broth mixture into a greased casserole dish.
Lightly toast the slices of bread. Place toasted bread in a single layer on top of onion/broth mixture.
Cover bread with a thick layer of the shredded mozzarella and top with grated Parmesan.
Place dish under the broiler, watching carefully. Broil until the cheese melts and begins to brown and bubble.
Let cool for 2-3 minutes. Serve immediately.