Pat the beef cubes dry with paper towels and season with salt and black pepper.
Heat 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 diced onion and cook for 3-4 minutes until softened. Add minced garlic and cook for 1 minute until fragrant.
Add sliced mushrooms to the skillet and cook for 5 minutes until they release their moisture and begin to brown.
Stir in tomato paste, cooking for 1 minute. Pour in red wine and beef broth, scraping up any browned bits from the bottom of the skillet.
Add Worcestershire sauce, dried thyme, and dried rosemary. Stir to combine and bring to a simmer for 2 minutes.
Pour the mixture over the beef in the slow cooker. Cover and cook on low for 6 hours or until the beef is tender.
About 30 minutes before serving, melt butter in a small bowl and whisk in flour until smooth to make a slurry.
Stir the slurry into the slow cooker to thicken the sauce. Cover and cook for an additional 30 minutes.
Taste and adjust seasoning with salt and pepper if needed.
Serve the beef and mushrooms hot, garnished with chopped fresh parsley.