Microwave onions, garlic, tomato paste, oil, and thyme in bowl, stirring occasionally until onions are softened, about 5 minutes; transfer to slow cooker.
Stir broth, mushrooms, carrots, celery, soy sauce, and bay leaves into slow cooker.
Season beef with salt and pepper and nestle into slow cooker.
Cover and cook until beef is tender, 9-11 hours on LOW or 5-7 hours on HIGH.
Transfer beef to a cutting board, let cool slightly, then shred into bite-size pieces.
Let soup settle for 5 minutes, then remove fat from surface using large spoon.
Discard bay leaves.
Cook egg noodles in boiling salted water until tender, then drain.
Stir cooked noodles and shredded beef into soup and let sit until heated through, about 5 minutes.
Stir in parsley, season with salt and pepper to taste, and serve.