Take the roast out of the packaging and pat dry with paper towels.
Cut onion into thick slices and place in the bottom of the slow cooker. Place roast on top of onions.
Season with salt, pepper, garlic, and thyme. Pour beef broth around the sides of the roast.
Put the lid on the crock pot and, cook on high for 6 hours or 8 hours on low.
Remove the roast from the slow cooker, and let it rest for 15 minutes. Place on a serving platter and spoon over drippings or gravy, then serve.
To Make Gravy
Skim off the fat and pour the dripping into a saute pan. Heat pan to medium-low and reduce liquid by ⅓. If needed, thicken the gravy with a slurry made with two tablespoons of flour and ¼ cup of cold water. Whisk the slurry into the drippings and bring it up to a simmer to thicken. Taste and adjust with salt and pepper.