Dahi Wali Macchi (Haddock Baked in Yoghurt)

Name: Dahi Wali Macchi (Haddock Baked in Yoghurt)
Prep time: 15 min
Cook time: 50 min
Total time: 50 min
Yield: 6 portions
Ingredients: 200 g onions; 600 g haddock fillets; 10 g fresh root ginger; 425 g plain yoghurt; 2 tsp ground cumin; 1 tsp sugar; 1 ½ tsp fine sea salt; ½ tsp cayenne pepper; ¼ tsp ground black pepper; ¼ tsp garam masala; 2 Tbsp ground coriander; 2 Tbsp lemon juice; 50 g vegetable oil; 40 g unsalted butter; 2 sprigs fresh coriander