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1
Adjust oven rack to lower-middle position and preheat oven to 325u00b0F.
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2
Season meat liberally with salt and pepper. Toss in flour to coat. Heat oil in a Dutch oven over medium-high heat until shimmering. Add short ribs and brown on all sides, about 7 minutes total.
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3
Roughly chop one onion and add it to the pan, along with the garlic, bay leaves, two sprigs of thyme, 7 cups of broth, Worcestershire sauce and red wine. Bring to a boil, and transfer to the oven to cook until meat is fork-tender, 2 to 2 1/2 hours.
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4
Remove meat from oven and transfer to a platter until cool enough to handle. Meanwhile, strain broth and discard solids. Remove and discard bones from meat. Shred meat and reserve.
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5
Thinly slice remaining 4 onions. Melt butter in a lidded saute pan over low heat. Add onions and remaining two sprigs of thyme to pan. Cover and sweat onions for 15 minutes, stirring occasionally, until soft. Remove cover, add sugar and raise heat to medium-high. Saute, stirring periodically so onions don't burn. Allow them to brow until deeply caramelized, about 7 minutes more. Discard woody stems from thyme, add apple and saute another 2 minutes. Add sherry vinegar and let onions absorb the liquid. Add sherry and bring to a boil, scraping up browned bits on the bottom of the pan.
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6
Transfer onion and apple mixture to the broth. Add remaining 3 cups of broth and simmer over low heat for 25 minutes to allow flavors to combine.
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7
Adjust oven rack to upper-middle position and preheat broiler on high setting. Divide the soup evenly between 6 ovenproof bowls. Add a generous handful of shredded meat and garlic bread, pressing it down into the broth so it absorbs liquid. Cover with enough of both cheeses, allowing them to hangs over the side of the crock.
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8
Transfer to the oven and broil until bubbly and browned, about 8 minutes. Serve immediately.
