Preheat your oven to 170F (77C).
Rinse the brisket and place it into a large oven-safe bag with the spice packet that came with the brisket.
Add the halved onion and garlic cloves to the bag.
Pour water into the bag and seal it.
Cook the brisket in the oven overnight.
The following morning, remove the brisket and place it into ice baths to cool.
Once cooled, remove the fat cap and all other fat running through the brisket.
Slice the brisket across the grain into 1/2-inch wide pieces.
Melt half the butter in a large pan over medium heat.
Add the onions to the pan and sauté until they're tender.
Add the shredded cabbage to the pan and cook until it's wilted.
Add the remaining ingredients, including the rest of the butter, and cook until the butter is well mixed in and the cabbage is tender.
In a large saucepan, melt bacon fat and add onions. Cook slowly for 10 minutes.
Add garlic and cook for another 3-5 minutes.
Add cabbage and cook down until soft and limp.
Add the remaining ingredients and continue to cook until all the flavors are well combined and the cabbage is tender.