Heat olive oil in a large frying pan and sear the roast on each side, for about 2 minutes per side, until nicely browned. Heavily salt and pepper both sides. Set aside.
In the same pan, add the fennel and shallots and cook on medium until tender, 4-5 minutes, stirring often. Add more oil if needed.
Spray the inside of slow cooker with cooking spray. Lay the red onion slices at the bottom. Place the roast beef on top. Place the cooked fennel and shallot on top of the beef. Next, lay celery on top.
Mix together the stock, red wine, Worchestersire sauce, and thyme. Generously add salt and pepper. Pour over the beef and vegetables. Salt and pepper again.
Cover and cook on HIGH for 4 hours. (Cook on LOW 6-8 hours:) The last hour of cooking time, lay the sweet potatoes on top. Lightly salt and pepper. Cook for one more hour; garnish and serve!