Dredge the roast in the flour, coating all sides and shaking off any excess.
Heat the olive oil in a large skillet or Dutch oven over medium-high heat.
Brown the roast on all sides, about 2 minutes per side.
Transfer the roast to a slow cooker sprayed with olive oil cooking spray.
Cut two small slits into the top of the roast and insert the mashed garlic cloves.
Place the chopped carrots, potatoes, and onion around the roast and season with garlic salt and pepper.
In a separate bowl, combine the stout, beef broth, and Worcestershire sauce,and pour it over the roast.
Add the bay leaf, prunes (if using), and sprigs of thyme.
Cook on Low for 8 hours or 4 hours on High then switch to Low for one more hour.
Garnish with fresh parsley and serve.