Heat half the oil in a large, heavy-based pan and fry the braising steak in batches over a medium-high heat until well browned. You will need to do this in batches. Remove with a slotted spoon and set aside.
Add the onion to the pan and cook for 5 minutes. Add the garlic, cumin, coriander seeds and chilli and cook for 3 minutes. Add the tomatoes, purée, stock and chocolate. Return the beef to the pan and stir just until mixed into the rest of the ingredients.
Bring to the boil, cover and simmer over a very low heat for 2 hours, adding the beans for the last 10 minutes. Season and stir in the coriander.
Serve spooned over the tortillas with the sour cream, avocado, tomatoes and lime wedges on the side.