Rub the beef chuck on all sides with salt, then rub with mustard and grated garlic.
Place the beef roast on a sheet tray fitted with a wire rack and store in the refrigerator for 8 to 12 hours.
When ready to cook, remove the beef chuck from the refrigerator. Heat the oil in a Dutch oven over medium heat.
Add the beef chuck and sear it on all sides until deeply browned, about 4 minutes per side. Remove and reserve.
Add the sausage slices and onion to the pot with the oil and cook until the sausage is browned and the onion is softened.
Add the beer, scraping any browned bits from the bottom of the pot. Simmer until reduced by half, about 5 minutes.
Add the beef broth, juniper berries, pepper, bay leaves, thyme, and rosemary. Tie the bay leaves, thyme, and rosemary together in a bouquet for easier removal.
Preheat the oven to 325 F.
Return the beef chuck to the pot and bring to a boil.
Cover and place in the oven. Roast for 2 hours.
Remove the Dutch oven from the oven, add the carrots, potatoes, and cabbage, cover, and return the pot to the oven.
Once the beef is pull-apart tender and the vegetables are softened, about 90 minutes longer, remove the pot from the oven.
Remove the beef roast from the pot.
Mix together the softened butter and flour until very well combined.
Bring the pot of vegetables and liquid to a simmer over medium heat. Add the butter mixture and stir to combine.
Simmer until thickened, about 4 minutes. Remove from the heat.