Easy-to-scale cheesy fish pie with kale

Ingredients

• 200g Maris Piper potatoes , cut into chunks
• 1 tbsp butter
• 1 tbsp flour
• 200ml whole milk , plus 1 tbsp
• ½ tsp Dijon mustard
• 100g cheddar , gruyère or emmental, grated
• 200g fish pie mix (we used a mix of salmon, haddock and prawns)
• 50g frozen peas
• 50g shredded kale
• 1 spring onion , finely sliced
• 1 tbsp parmesan , finely grated