Season the roast with salt, pepper, and thyme. Heat the olive oil in a large skillet over medium-high heat. Place the meat in the skillet and sear it on all sides until it's brown all over. Transfer the meat to a slow cooker.
Add onions, garlic, thyme, bay leaf, broth, and beer. Turn on high and cook, covered, for 3 hours.
Remove the lid and add carrots and cornstarch mixed with meat juices into a paste (prevents from clumping up). Cook on low for another hour or longer, depending on your slow cooker. The roast is ready when it's fall-apart tender.
I prefer my potatoes cooked separate but you can add 4 cups of chopped potatoes to the slow cooker at the same time as carrots.