Heat oil in a small saucepan over medium heat for a minute. Add fennel and cook, stirring for 5 minutes or until the fennel is lightly caramelised. Transfer the fennel to the slow cooker.
Return the saucepan to the heat and cook diced beef, in batches, to brown on all sides. Transfer each batch to the slow cooker.
Add bone broth to the saucepan to deglaze the pan and transfer the broth to the slow cooker as well. Sprinkle with then stir in the cacao and chocolate.
Top the dish with diced sweet potato, cover with the lid and cook on high for 4 hours or low for 8 hours.
To serve, divide the casserole between 4 serving plates, sprinkle with cacao nibs for extra flavour and crunch, and some chopped parsley.