Place the roast in a nonreactive bowl or other container just wide enough to hold it and add enough whey to cover. Transfer to the refrigerator to marinate for 24 hours. This will tenderize the meat. 

When ready to cook, remove the meat from the whey (discard whey) and return the meat to room temperature. Using the tip of a sharp paring knife, make 20 evenly spaced small slits about 1 1/2 inches deep all over the pot roast.

Using your fingers, insert the garlic cloves as deep into the meat as possible. Season the roast on all sides with the salt and pepper. 

Heat a large skillet over medium-high heat. Add the oil and, when hot, sear the meat on all sides until very well browned, about 4 to 6 minutes per side. When the roast is evenly browned on all sides, transfer to a slow cooked. Carefully add the water or beef broth, scrape the bottom of the pan to deglaze and then transfer the liquid to the slow cooker. Cover and set the slow cooker to high for 6 to 8 hours.

Reduce the heat to low/medium-low and transfer to the oven. Continue to cook the roast until it is very tender, about 3 to 3 1/2 hours, turning 2 or 3 times during the entire cooking time. Check occasionally to make sure that there is always some liquid in the pan. If necessary, add more water. 

When the roast is very tender, carefully transfer to a serving platter with a meat fork. Discard rosemary sprigs. Transfer the cooking liquid to a small saucepan over medium heat.

In a small bowl, make a paste with the butter and flour and whisk some of the hot juices from the saucepan into the butter-flour mixture. Whisk the mixture back into the remaining hot juices in the pan and bring to a simmer on the cooktop. Cook until the juices are thick enough to coat the back of a spoon and any floury taste is gone. Season, if necessary, with salt and pepper. 

Slice or pull meat apart into serving pieces and serve with the gravy drizzled over the top. Serve with Creamy Polenta and Braised Broccoli Rabe, if desired.