Vegetarian chilli tacos

Name: Vegetarian chilli tacos
Ingredients: 1 tbsp olive oil; 1 onion, finely chopped; 1 garlic clove; 3 celery stalks, finely chopped; 200g/7oz portobello/shiitake mushrooms, chopped; 2 green chillies, chopped; 100g/3½oz tomato purée; 1 tsp cocoa powder (optional); 150ml/5fl oz vegetable stock; 2 x 400g tinned kidney beans, or mixed beans, rinsed and drained; salt and freshly ground black pepper; 12 ready-made taco shells, warmed; 150g/5oz Monterey Jack (or mild cheddar) cheese, grated; 2 tbsp chopped fresh coriander (optional); 200ml/7fl oz plain yoghurt