Step 1

Melt butter in a large saucepan over low heat. Add onions and cover. Cook for about 30 minutes, stirring occasionally.

Step 2

Add the beef broth and the Worcestershire sauce. Heat the broth mixture until it boils.

Step 3

Reduce heat to low. All to simmer, uncovered, for about 30 minutes. The broth will reduce.

Step 4

Pour onion and broth mixture into a greased casserole dish.

Step 5

Lightly toast the slices of bread. Place toasted bread in a single layer on top of onion/broth mixture.

Step 6

Cover bread with a thick layer of the shredded mozzarella and top with grated Parmesan.

Step 7

Place dish under the broiler, watching carefully.  Broil until the cheese melts and begins to brown and bubble.

Step 8

Let cool for 2-3 minutes. Serve immediately.