Preheat the grill to high. Put the potatoes in a pan of boiling water and cook for 10 minutes. In a second pan of boiling water cook the egg for 8 minutes.
Meanwhile, heat a little butter in a pan and gently fry the onion until soft. Stir in the crème fraiche and when the sauce is coming up to the boil, add the haddock pieces and simmer for 5 minutes until it is cooked through.
Drain the potatoes then rinse the egg in cold water before peeling and cutting into quarters. Mash the potatoes with butter, season, then set to one side.
Add the pieces of egg, herbs and mustard to the sauce and season with ground black pepper, then pour into an ovenproof dish. Top with the mashed potato, using a fork to rough up the top. Grill for 4-5 minutes until golden and bubbling then serve immediately on warmed plates.
The fish pie doesn't need to go in the oven, which makes it super-fast to cook. Make ahead for a relaxed midweek supper.