Curried swede soup

Name: Curried swede soup
Ingredients: 2 tbsp olive oil; 30g butter; 1 onion, sliced; 2 celery sticks, finely chopped; 1 thumb-sized piece of ginger, peeled and finely grated; 1 large garlic clove, crushed; 1 tsp ground cumin; 1 tsp ground coriander; 1 tsp ground turmeric; 1 tbsp medium curry powder; 800g swede, peeled and chopped into 3cm pieces; 1.8 litre vegetable or chicken stock; 50ml double cream; yogurt and chilli flakes, to serve; 180g mature cheddar, coarsely grated; 3 spring onions, finely sliced; 1 heaped tsp nigella seeds; ½ small bunch of coriander, finely chopped; 2 tbsp butter; 4 large slices white sourdough; 2 tbsp mango chutney