01 - Season the beef pieces evenly with kosher salt and freshly ground black pepper.
02 - Heat a large skillet over medium-high heat and sear beef on all sides until browned, about 5 minutes; transfer to slow cooker.
03 - Add onion, carrots, celery, and garlic to the skillet and sauté for 3 to 4 minutes until softened; transfer to slow cooker.
04 - Pour crushed tomatoes, tomato paste, red wine, and beef broth into slow cooker. Stir in dried oregano, basil, thyme, bay leaf, and crushed red pepper flakes if using.
05 - Cover and cook on LOW for 8 hours until beef is very tender and shreds easily.
06 - Remove bay leaf and shred beef with two forks directly in slow cooker.
07 - Stir in unsalted butter and chopped fresh parsley. Adjust seasoning to taste.
08 - Serve sauce hot over cooked pasta or creamy polenta and garnish with grated Parmesan cheese.