Chilli con carne soup

Name: Chilli con carne soup
Cook time: 1 h 30 min
Total time: 1 h 30 min
Cuisine: American, Latin American, TexMex
Ingredients: 1 small onion , finely chopped; 2 garlic cloves , finely chopped; 1 tbsp vegetable oil; 500g lean minced beef; 410g can pinto or red kidney bean , drained; 400g cans chopped plum tomatoes; 700ml/1¼ pints hot chicken stock; large pinch of crushed dried chillies; 2 squares of dark chocolate (try Lindt 85%); fresh coriander or parsley leaves and some grated gruyère, to serve