In a large bowl, blend pepper, salt, and flour together until combined. Add stew beef and toss to coat.
Add beef to hot skillet and sear until browned all over, about 4-5 minutes. You may have to do this in batches so you don’t crowd the pan. Remove seared beef and add to your slow cooker. Leave behind the juices and brown bits.
Add onions to the skillet and sauté until softened, about 4-5 minutes. Add carrots, celery, garlic, and sauté 4-5 minutes more.
Add 1 cup of beef broth to the skillet to deglaze the pan. As the broth bubbles and simmers, scrape the skillet with a wooden spoon to release all the flavor infused brown bits stuck to the bottom of the pan. Turn off heat and transfer everything in the skillet to your slow cooker.
Add the remaining ingredients to the slow cooker: potatoes, Worcestershire sauce, sugar, paprika, bay leaves, thyme, and enough beef broth to cover. Stir to combine.
Cover, and cook on low for 7-8 hours, or on high for 4-6 hours, stirring occasionally until the beef is tender. Remove bay leaves and thyme sprigs before serving. Taste and adjust seasoning, if needed.