Place the fish in a large pan. Pour in the milk and water. Add the bay leaf. Simmer gently for about 5 minutes until the fish is flaky. Remove the fish and set aside.

Pour the poaching liquid through a sieve into a bowl. Keep this liquid as it holds all the smoky flavor. Discard the bay leaf. Wipe the pan clean to use it again for the vegetables.

Melt the butter in the same pan over medium heat. Add the chopped onions. Cook them slowly for about 5 to 8 minutes. They should be soft and translucent, not brown or crispy.

Add the cubed potatoes to the pan with the onions. Pour in the saved poaching liquid. Bring the mixture to a gentle boil. Lower the heat and simmer until the potatoes are very soft.

While the potatoes cook, remove the skin from the haddock. Use your fingers or a fork to flake the fish into large chunks. Check carefully for any small bones and remove them immediately.

Once the potatoes are cooked, use a masher to crush some of them. Do not mash everything. You want some chunks for texture and some mashed to thicken the beautiful, creamy soup base.

Stir the flaked fish back into the pan. Add the double cream if you are using it. Let everything heat through for 2 minutes. Taste it first before adding salt, as smoked fish is salty.

Ladle the thick soup into warm bowls. Sprinkle fresh chives or parsley on top. Serve it immediately with crusty bread and plenty of butter. Enjoy your homemade Scottish classic with your family.