Preheat the oven to 190°C. Spray a frying pan with low-calorie cooking spray and set on a medium heat. Add the onions and fry for 5 minutes. Add the garlic and continue to fry for 2 minutes.
Add the Dijon mustard and pour in the milk, season with salt and pepper. Place the fish on top and cook for 5 minutes until the fish is just starting to break up. Combine the cornflour and water in a small mixing bowl and pour into the pan. Give the pan a gentle stir to break up the fish and incorporate the cornflour mixture.
Stir through the peas and pour the fish pie mixture into the ovenproof dish.
In a small bowl combine the Primula cheese and milk and whisk until smooth. Season with salt and pepper.
Place the cauliflower florets on top of the fish pie filling. Spoon the cheese mixture over the top of the cauliflower. Sprinkle over the grated cheese and pop into the oven for 25 minutes, until the cheese is golden brown and cauliflower cooked.
Sprinkle over the chopped chives and serve.