1. Preheat oven to 220°C (425°F). Grease a 23x33 cm (9x13 inch) baking dish with butter

2. Place potatoes in a large pot cover with cold water and add a pinch of salt. Bring to a boil and cook until tender about 15-20 minutes

3. Meanwhile cook bacon in a skillet over medium heat until crisp. Remove half the bacon with a slotted spoon and set aside for garnish. Add onion to the skillet and cook until softened about 5 minutes. Remove from heat

4. Drain potatoes well and return to the pot. Mash with butter salt and pepper until smooth. Stir in egg yolks

5. In a medium bowl whisk together heavy cream and flour until smooth. Stir in sour cream Dijon mustard parsley and the bacon mixture. Season with salt and pepper

6. Pipe or spoon the mashed potatoes around the outer edge of the prepared baking dish to form a border

7. Place cod fillets in the center of the dish inside the potato border

8. Pour the cream mixture evenly over the cod

9. Sprinkle grated cheese over the top

10. Bake for 25-30 minutes until the fish is cooked through and the sauce is bubbling

11. Switch oven to broil and broil for 2-3 minutes until the cheese and potato edges are golden and crisp. Watch carefully to avoid burning

12. Garnish with the reserved crispy bacon