Mackerel Fillets Baked in Cider

Name: Mackerel Fillets Baked in Cider
Ingredients: 2 good-sized mackerel fillets, boned and halved lengthwise; salt; 1 heaped tablespoon mustard powder, blended with 1 tablespoon of water; 2 medium-sized onions, sliced thinly; 2 granny smith apples, sliced; 1/2 medium-sized turnip/swede/rutabaga, sliced thinly; Cajun spice mix; Cayenne pepper; black pepper; some fresh coriander, chopped; 250ml cider; 2 medium sized potatoes, sliced very thinly; butter