Place onion in a 4 quart electric slow cooker. Combine salt and pepper, rub on all sides of roast. Cook roast in hot oil in a skillet over medium high until browned on all sides.
Remove roast from skillet, for even cooking cut roast in half before placing it on top of onions in slow cooker. Add worcestershire sauce to skillet, deglazing skillet by scraping particles that cling to bottom. Pour worcestershire sauce mixture over meat and onion.
Cover and cook on high setting 1 hour. Reduce to low setting, and cook 7 hours or until meat is very tender. Remove meat and onion to a platter; cover and keep warm. Combine flour and water, stirring until smooth.
Stir flour mixture into drippings in cooker. Cook, uncovered, on high 10 minutes or until thickened and bubbly, stirring occasionally. Makes 8 servings.