Heat olive oil in a large skillet over medium-high heat.
Add beef cubes and brown on all sides, about 4-5 minutes per batch. Transfer browned beef to the slow cooker.
In the same skillet, add diced onion and cook until softened, about 3 minutes.
Add minced garlic and cook for 1 minute more.
Transfer onion and garlic to the slow cooker.
Add beef broth, water, tomato paste, Worcestershire sauce, dried thyme, dried rosemary, black pepper, and salt to the slow cooker. Stir to combine.
Add sliced carrots and celery to the slow cooker.
Cover and cook on low for 6 hours.
After 6 hours, add the egg noodles to the slow cooker and stir.
Cover and cook on high for 30 minutes, or until noodles are tender.
In a small bowl, whisk together cornstarch and cold water until smooth.
Stir cornstarch mixture into the slow cooker to thicken the broth.
Cook uncovered on high for 5-10 minutes until broth is thickened.
Stir in chopped fresh parsley before serving.