Prep Time : 10 minutes
Cook Time : 6-8 hours  
Yield : 8

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Ingredients:

3 large onions, sliced (3 cups)
3 tablespoons margarine or butter, melted
3 tablespoons all-purpose flour
1 tablespoon Worcestershire sauce
1 teaspoon sugar
1/4 teaspoon pepper
4 cans (14 1/2 ounces each) ready-to-serve beef broth
8 slices French bread, 1 inch thick
3/4 cup shredded mozzarella cheese (3 ounces)
2 tablespoons grated or shredded Parmesan cheese

Directions: 

In the crock pot, mix onions and margarine.
Cover and cook on high heat setting 30 to 35 minutes or until onions begin to slightly brown around edges.
Mix flour, Worcestershire sauce, sugar and pepper.

Stir flour mixture and broth into onions.

Cover and cook on low heat setting 6 to 9 hours (or high heat setting 3 to 4 hours) or until onions are very tender.

Prepare slices of bread with cheeses and broil in the oven until bread is toasted and cheese is beginning to brown and bubbling.

Place 1 slice bread on top of each bowl of soup. Serve immediately.

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My notes:  

I found this recipe can be prepared as outlined above but it is better if you use a frying pan to soften the onions first.  

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Found on:  bettycrocker.com