Season the beef cubes with salt and pepper. Heat the olive oil in a large skillet over medium-high heat and brown the beef in batches until all sides are seared. Transfer the beef to the slow cooker.
Add the diced onion and quartered mushrooms to the same skillet. Cook for 5 minutes until the onion softens and the mushrooms release some liquid.
Stir in the minced garlic, tomato paste, and flour. Cook for 1 minute, then pour in the red wine. Scrape the bottom of the skillet to loosen any browned bits and let the mixture simmer for 2 minutes.
Transfer the skillet contents to the slow cooker. Add the carrots, beef broth, dried thyme, and bay leaves. Stir everything together.
Cover and cook on low for 8 hours. Remove the bay leaves before serving.