Heat the olive oil in a large skillet over medium-high heat.
Add the 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 the diced onion and cook for 3 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Add the sliced mushrooms, carrots, and celery to the skillet and cook for 5 minutes until vegetables begin to soften.
Transfer the vegetables to the slow cooker with the beef.
Pour the beef broth and red wine into the slow cooker.
Stir in the tomato paste, dried thyme, dried rosemary, and bay leaf.
Season with salt and black pepper to taste.
Cover and cook on low for 7 hours, or until the beef is tender.
About 30 minutes before serving, mix the all-purpose flour with cold water until smooth to create a slurry.
Stir the slurry into the slow cooker to thicken the stew.
Replace the lid and cook on high for 30 minutes until the stew thickens.
Remove the bay leaf before serving.
Garnish with chopped fresh parsley and serve hot.