Crispy Wasabi Prawn Sandwich (Wasabi Ebi Katsu Sando)

Ingredients

• 250g raw prawns
• ¼ onion, finely chopped
• 1 tbsp cooking oil
• 1 tbsp potato flour
• ½ tsp salt
• ½ tsp black pepper
• ½ tsp S&B Wasabi paste
• ½ cup flour
• 1 egg, beaten
• ½ cup panko breadcrumbs
• Oil for deep frying
• 1 egg
• 4 tbsp Japanese mayonnaise
• ¼ tsp salt
• ¼ tsp sugar
• ¼ tsp black pepper
• ¾ tbsp rice wine vinegar
• 4 slices white bread
• 1 cup shredded cabbage