BBC Food - Recipes - Spiced mackerel with horseradish potatoes

Ingredients

• 500g/1lb 2oz new potatoes , cooked in their skins
• 2 tbsp olive oil
• sea salt and freshly ground black pepper
• 2 shallots , finely sliced
• 2 tbsp soured cream
• 2 tbsp creamed horseradish
• 2 tbsp finely chopped fresh chives
• olive oil
• watercress leaves, to serve
• 8 mackerel fillets
• 100g/4oz butter , softened to room temperature
• 1 tsp cayenne pepper
• 2 tsp sweet smoked paprika
• 1 tsp ground coriander
• 1 tbsp golden caster sugar
• ½ tsp English mustard
• few drops Tabasco
• 2 tsp red wine vinegar