Season the chuck roast. Season all sides of your chuck roast with a generous pinch of salt and pepper.
Brown the chuck roast. Add the olive oil and butter to large skillet over medium-high heat. Once hot add the roast to the skillet and brown for approximately 4-5 minutes on each side.
Prepare the vegetables. As the chuck roast is browning, prepare the onions, celery, carrots, potatoes, mushrooms, and garlic and transfer to the slow cooker. Mix well to combine.
Transfer the beef and cook the pancetta. Once all sides of the chuck roast have browned, transfer the roast to the slow cooker and wedge between the vegetables. Return the skillet to medium heat and add the pancetta to the skillet. Cook the pancetta for 3-4 minutes, or until fully cooked.* Transfer the pancetta to the slow cooker.
Add the spices and broth. To the slow cooker add the salt, pepper, rosemary, thyme, Worcestershire sauce, Coca-Cola, beef broth, and bay leaf.
Cook. Cover and cook on low for 8-10 hours or on high for at least 5-6 hours.
Thicken. One hour before serving mix the cornstarch with the water in a small bowl. Add the cornstarch mixture to the slow cooker and gently mix to combine. Alternatively, just before serving, you may remove the chuck roast and vegetables from the pot to a large serving plate and transfer the liquid to a small pot over medium heat. Add the cornstarch-water mixture and cook until thickened, approximately 2-3 minutes.
Serve. Serve the meat with vegetables and gravy and garnished with fresh parsley, if desired.