Sear the Beef: Pat the beef chunks dry and season with salt and pepper. In a large skillet, heat 1 tbsp of olive oil over medium-high heat. Brown the meat in 2 batches (about 5 minutes per batch), adding the remaining oil for the second batch. Transfer the browned meat to the slow cooker.
Sauté Aromatics: In the same skillet, add the onions, celery, mushrooms, garlic, and balsamic vinegar. Cook for 5 minutes, scraping the bottom of the pan to loosen the browned bits.
Build the Base: Add the tomato paste and cook for 1 minute. Sprinkle the flour over the vegetables and stir for 1–2 minutes until the flour is incorporated.
Deglaze: Slowly pour in the red wine, stirring to combine. Simmer for 2 minutes, then stir in the beef broth, sugar, thyme and bay leaf.
Combine in Slow Cooker: Pour the liquid and vegetable mixture over the beef in the slow cooker. Add the carrots and potatoes, stirring gently so they are mostly submerged.
Slow Cook: Cover and cook on Low for 8 hours or High for 4 hours, until the beef is fork-tender.
Finish: Discard the bay leaf. Taste for seasoning and garnish with fresh parsley.