Preheat the oven to 200C/400F. Place the potatoes on a large baking tray and coat in a drizzle of oil and a good pinch of salt. Place in the oven for 1 hour, or until deep golden, crispy and knife tender.
Once cool enough to handle, slice off the top (3/4 of the way up) and scoop out most of the centre into a large bowl, leaving a thin wall to keep the potato sturdy. Mash the potato with butter, cream, mustard, cheese and plenty of salt & pepper to taste (I typically use 1/2 tsp salt & 1/4 tsp pepper).
Meanwhile, melt 2 tbsp butter in a large pan over medium heat. Fry the leek until soft and just starting to take on colour, then stir in 2 tbsp flour. Gradually add in the milk and cream, stirring as you go to ensure no lumps form. Stir in the chives, parsley, Oxo, nutmeg, pepper and salt (I typically do around 1/8-1/4 tsp salt but work to taste - just make sure the sauce is well-seasoned). If the sauce needs to thicken up just gently simmer until it does (see video for reference). Turn off the heat and gently fold in the fish, ensuring it doesn't break up too much.
Place the potato skins back on the baking tray and spoon in the filling. Top with a few heaped tbsp of the mash (you likely won't need it all, save the rest for a rainy day). Place back in the oven at 200C/400F for 20 minutes, or until the mash is golden and crisp and the sauce is bubbling around the edges. Timings will depend on the size of the fish pieces - it should be opaque, flaky and piping hot. You can peak under the mash if you want to check.