Pickled herring with potatoes and curried mayonnaise - Peter's Yard

Ingredients

• Approximately 12 small to medium new potatoes, boiled until tender but still firm enough to slice
• 5 very generous tbps mayonnaise
• 3 tbps crème fraiche
• 2 tsp Dijon mustard (or French’s mustard)
• ½ tsp of caster sugar, optional
• 4 pods green cardamom, de husked, shells discarded and seeds very finely ground with a pestle and mortar
• 2 tsp mild curry powder
• ½ tsp turmeric powder
• Few drops of white wine vinegar
• 1 jar of sweet pickled herrings, drained
• Half a medium red onion sliced thinly , crossways not root to top, so that you have semi circles of onion
• Salt