In a large skillet, heat the olive oil over medium-high heat. Add the beef pieces and brown them on all sides, about 5-7 minutes. Transfer the browned beef to the slow cooker.
In the same skillet, add the chopped onion and garlic. Sauté for 2-3 minutes until the onion is translucent. Add to the slow cooker.
Pour the beef broth over the beef and onion mixture. Add the sliced carrots, chopped celery, thyme, rosemary, salt, and pepper. Stir to combine.
Cover and cook on low for 7-8 hours or on high for 4-5 hours, until the beef is tender.
About 30 minutes before serving, cook the egg noodles according to package instructions. Drain and set aside.
In a small bowl, mix the cornstarch and water to create a slurry. Stir this into the slow cooker to thicken the sauce. Cook for an additional 30 minutes.
Serve the beef mixture over the cooked egg noodles and garnish with chopped parsley if desired.