Season beef cubes well with salt and pepper. Heat oil in a large skillet or Dutch oven over medium-high heat. Add beef cubes and sear on all sides until golden-brown. Transfer beef to a 4-qt slow cooker.
Add onion, carrots, and celery to pan drippings and sauté over medium-high heat until onions are translucent. Add garlic and cook 1 minute until fragrant.
Stir red wine into vegetables using a spoon to scrape up any browned bits. Add tomato paste, beef broth, and crushed tomatoes and bring to a simmer, cooking until everything is combined.
Top beef cubes with sauce and stir well to combine. Add bay leaf and thyme sprigs.
Top slow cooker with lid and cook beef on high 4-5 hours or on low 7-8 hours until beef is fall-apart tender and sauce is slightly reduced.
Remove bay leaf and thyme sprigs from slow cooker and discard. Use two forks to pull beef apart into bite-sized pieces, then add back to sauce. Season beef ragu with salt and pepper to taste.
To serve, ladle beef ragu over pappardelle pasta and top with grated Parmesan and fresh parsley. Enjoy!