In a sauté pan over a low to medium heat, poach the haddock in the milk with the bay leaf for 3 to 4 minutes until cooked. Remove the haddock and pour the milk into a separate bowl and set aside. Discard the bay. Wipe the pan clean of milk with a towel.
In the same pan, sauté the onion and leek in butter until soft, but stopping before they're browned. Add the potato, coating it well in the buttery vegetables.
Add the retained milk and simmer until the potato is cooked.
Remove half the vegetables and mash the milky vegetable mix with a potato masher or fork. Add back the remaining vegetables and haddock to warm through.
Check for seasoning and serve with a good grinding of black pepper.