Slow cooker chilli con carne

Name: Slow cooker chilli con carne
Ingredients: 3 tbsp olive oil; 500g beef mince; 1 onion, finely chopped; 1 celery stick, finely chopped; 1 large red pepper, thickly sliced; 2 garlic cloves, finely grated; 2 tsp ground cumin; 1 tsp smoked paprika; 2 tsp dried oregano; 2-3 tbsp chipotle chilli paste; 3 tbsp tomato purée; 400g can chopped tomatoes; 400ml beef stock; 2 x 400g can black beans, drained; 4 small squares dark chocolate; tortilla chips or cooked rice and soured cream, to serve