Cut onions in half and slice into thin strips. Heat medium soup pot over medium-high heat.
Add butter to heated pan and once butter is melted, add onions, sugar, and olive oil. Cook for 20-25 minutes, stirring regularly, until onions are caramelized.
Once onions are caramelized, add flour and stir. Add bone broth, salt, pepper, and bone broth. Stir well to ensure onions are not stuck to bottom of pot.
Increase heat and bring to a boil. Once soup is boiling, reduce to low and simmer for 30 minutes.
Pour soup into oven-safe bowls top with croutons or baguette and parmesan cheese. Place under oven broiler on high for 4-5 minutes until cheese is melted and slightly browned.
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