Turkey curry

Name: Turkey curry
Ingredients: 1 tbsp olive oil; 25g/1oz unsalted butter; 1 large onion, peeled and finely chopped; 4 garlic cloves, peeled and finely chopped; 2.5cm/1in knob of fresh ginger, peeled and grated; 1 red chilli, de-seeded and finely chopped; 8 green cardamom pods, slightly crushed; 1 tsp ground cumin; 1 tbsp ground turmeric; ½ tsp chilli powder; 1 tsp garam masala; 1 tsp ground coriander seeds; 2 large potatoes, peeled and cut into cubes; 1 butternut squash, peeled, seeds removed and cut into cubes; 570ml/1pt chicken or turkey stock; 125ml/4fl oz yoghurt; 85ml/3fl oz double cream; 1 tbsp lemon juice; 6 large handfuls roast turkey, chopped; 1 tbsp fresh coriander leaves, chopped