Carmelize the onions in a large pan on medium-low. About 20-30 minutes. Remove from pan.
Season the bricket liberally with salt and pepper on both sides. Turn the heat up to medium high and sear the brisket on all sides.
Place the onions in a slow cooker, place the brisket on top, cover with the garlic and add the rest of the liquid ingredients. Slow cook on high for 4-5 hours.
When there is about 40 minutes left on the brisket, put the chopped potatoes in a large pot, cover with water, salt the water and boil for 20-25 minutes.
While the potatoes boil, sautxe9 the onions for the cabbage in butter and garlic.
When the onions have softened add half the cabbage, toss to coat with butter. Continue to cook, covered, on medium low stirring occasionally until cabbage softens- about 10 minutes.
Add the rest of the cabbage to the pan, toss to coat, cover and cook another 10 minutes.
Drain potatoes, return the pot to the heat, reduce to medium, add butter and garlic and add kale. Cover the pot pot and let cook til kale has fully wilted- a good 8-10 minutes.
Return potatoes to the pot, add butter, cream, sour cream, salt, pepper and mash to desired consistency.
Remove the brisket from the slow cooker, strain the juices into a bowl, separate the onions.
In a medium sized pot, melt the butter and whisk in flour to create a roux, then add the brisket juices and whisk til smooth. Allow to simmer for 5 minutes, then remove from heat.