Heat olive oil in a large skillet over medium-high heat.
Add beef cubes in batches and brown on all sides, about 3-4 minutes per batch. Transfer browned beef 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 onions and garlic to the slow cooker.
Add sliced mushrooms, carrots, and celery to the slow cooker.
In a bowl, whisk together beef broth and tomato paste until combined.
Pour the broth mixture over the ingredients in the slow cooker.
Add dried thyme, dried rosemary, bay leaf, salt, and black pepper to the slow cooker. Stir gently to combine.
Cover and cook on low for 6 hours or until beef is tender.
About 30 minutes before serving, mix cornstarch and cold water in a small bowl to create a slurry.
Stir the slurry into the slow cooker to thicken the stew.
Cover and cook on high for an additional 30 minutes until the stew has thickened.
Remove bay leaf and adjust seasoning with salt and pepper if needed.
Sprinkle chopped fresh parsley over the stew before serving.