Crockpot French Onion Soup

6 large onions, chopped
1/2 cup butter
60-65 ounces condensed beef broth (undiluted)
1 1/2 tsp Worcestershire sauce
3 bay leaves
10 slices French bread, toasted
Shredded mozzarella cheese
Shredded parmesan cheese

In a large skillet, saute onions in butter until crisp-tender. Transfer to an ungreased 5 qt (or larger) crockpot. (I only had a 4 quart, so I made the leftover soup in a regular stove-top pot.) Add broth, Worcestershire, and bay leaves. Cover and cook on low for 5-7 hours or until the onions are tender. Discard the bay leaves. Top each serving with French bread and cheeses.