Unwrap and remove any twine from the beef roast. Measure seasonings into a small bowl; stir to combine. Sprinkle seasoning liberally on all sides of the roast.
Add oil to skillet over medium-high heat. **If your slow cooker has a sauté option, you can use that.When oil is shimmering, add it to the skillet and brown on all sides for 2-3 minutes each, or until a golden brown crust develops. (See Notes)
Layer half of the onions in the bottom of the crockpot. Place browned roast over the onions, then layer the other half of the onions and the rosemary sprigs (if using) on top of the meat. Combine the Worcestershire sauce and beef broth, then pour the liquid around the roast in the bottom of the slow cooker.
Cook on LOW heat for 8 hours. When done, the roast should be fork tender (you should be able to twist a fork in it) and the internal temperature at the thickest portion of the beef should reach at least 165ºF.
Remove the sirloin roast from crockpot and transfer it to a carving board or serving platter. Tent the meat with a sheet of aluminum foil and allow it to rest for 10 minutes before serving.
Notes
Browning the roast is optional, but I feel it gives added flavor to this recipe.To make gravy: Mix 2 tablespoons of cold water and 2 tablespoons of cornstarch in a small bowl. Combine it with the juices from the meat and a pinch of salt (to taste). Whisk this combination together over medium-high heat (in a skillet or your slow cooker if it has a saute option) until it comes to a boil, then simmer until thickened. Serve with your slow cooker sirloin tip roast or over mashed potatoes or vegetables.