In a large nonstick skillet, heat the oil until shimmering. Season the chuck roast on all sides with the seasoning mix. Sear the meat in the hot oil, turning to brown all sides. Remove the meat to the crock of a 5-qt slow cooker.
In the drippings, cook the onion and mushrooms until tender. Add the vegetables to the slow cooker and add the beef broth. You can also store the sautéed vegetables in the refrigerator until serving time, reheating before serving.
Set the slow cooker to LOW and cook for 8 hours or on HIGH for 6 hours.
When the meat is cooked, remove the jus from the crock with a ladle or baster. Shred the meat and combine it with the vegetables in the crock. Adjust seasonings and moisten with jus if needed. If freezing, wrap the meat-vegetable mixture and the jus separately. Thaw and reheat to serve.
In a small bowl, combine the butter, garlic, and parsley. Spread this on the insides of all the rolls. If freezing, wrap the rolls in foil and freeze. Thaw before serving.
To serve, broil the garlic bread rolls until golden brown and crisp. Fill the rolls with the meat mixture. Serve with the jus on the side.
If you're eating low-carb, paleo, Whole 30, or gluten-free, this luscious, meat-mushroom-and-onion filling is great on lettuce wraps. Add a few tomatoes to give it a little extra color and pizazz.