Crisp Bacon: In a large heavy skillet or Dutch oven over medium heat, fry the bacon lardons for 5 to 7 minutes until crisp and golden brown. Using a slotted spoon, transfer the bacon to your 6-quart slow cooker, leaving the rendered bacon fat in the skillet.

Sear Beef in Batches: Pat the cubed beef chuck roast thoroughly dry with paper towels. Season all sides with kosher salt and black pepper, then dust lightly with the flour. Increase skillet heat to medium-high. Sear the beef cubes in 2 or 3 uncrowded batches for 3 to 4 minutes per side until deeply browned with a rich dark crust. Transfer browned beef to the slow cooker.

Deglaze the Skillet: Reduce heat to medium. Add tomato paste and minced garlic to the skillet drippings; cook for 1 minute until fragrant. Pour in the red wine, scraping up all the savory browned fond from the bottom of the pan with a wooden spoon. Bring to a rapid boil for 2 minutes to cook off raw alcohol.

Assemble & Slow Cook: Pour the simmering wine reduction over the beef and bacon in the slow cooker. Add the beef bone broth, sliced carrots, Worcestershire sauce, fresh thyme sprigs, rosemary, and bay leaves. Stir gently to combine. Cover and cook on LOW for 7 to 8 hours (or on HIGH for 4 to 5 hours) until the beef is fork-tender and meltingly succulent.

Sauté Mushrooms & Onions: In the final 30 minutes of slow cooking, melt 2 tablespoons of butter in a skillet over medium-high heat. Add the quartered cremini mushrooms and pearl onions. Sauté for 6 to 8 minutes until golden-brown and caramelized. Stir the sautéed mushrooms and onions into the slow cooker for the last 20 minutes.

Thicken & Serve: Remove herb stems and bay leaves. If you prefer an extra-thick gravy, ladle ½ cup of braising liquid into a small bowl, whisk with 1.5 tablespoons cornstarch, stir back into the slow cooker, and cook on HIGH for 10 minutes uncovered until glistening and rich. Garnish generously with fresh parsley and serve steaming hot over garlic mashed potatoes!