Heat olive oil in a large skillet over medium-high heat.
Add the beef stew meat in batches and brown on all sides, about 3-4 minutes per batch. Transfer browned meat to the slow cooker.
In the same skillet, add diced onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Transfer the onion and garlic to the slow cooker.
Add sliced carrots, celery, beef broth, water, Worcestershire sauce, dried thyme, dried rosemary, bay leaf, salt, and black pepper to the slow cooker.
Stir to combine all ingredients.
Cover and cook on low for 6 hours or until beef is tender.
About 30 minutes before serving, cook egg noodles according to package instructions until al dente. Drain and set aside.
In a small bowl, whisk together all-purpose flour and cold water until smooth to make a slurry.
Remove the bay leaf from the slow cooker.
Stir the flour slurry into the stew to thicken the sauce.
Cover and cook on high for an additional 20-30 minutes until the stew thickens.
Serve the stew over the cooked egg noodles and garnish with chopped fresh parsley.