1. Add grapeseed oil and butter to a large Dutch oven or heavy bottom pot and heat over medium high until butter is melted. 
2. Add the sliced onions, kosher salt, and brown sugar and stir to coat. 
   
3. Reduce heat to medium and stir about every 5 minutes until all the moisture evaporates from the onions.
4. At this point, you will need to stir more frequently to prevent uneven caramelization or burning.
5. Continue to stir and cook the onions for 45 minutes to an hour until you have achieved a nice dark brown caramelization. 
6. Stir in the minced garlic and cook until aromatic, about 30 seconds.
7. Deglaze pan with sherry and balsamic vinegar being sure to scrape up any brown bits from the bottom of the pot.
8. Stir in flour and cook for about a minute, stirring continuously
9. Add beef broth, fresh thyme, and bay leaf, and simmer for 20 minutes.
   

Topping (see notes section if you don’t have cocottes)

1. Ladle the soup into oven proof ramekins or cocottes, top each cocotte with a slice of crusty bread and shredded cheese.
2. Place cocottes on baking sheet and place under the broiler until the cheese becomes golden and bubbly.