Melt the butter with olive oil in a large heavy pot over medium heat until smooth and shimmering

Add the chopped onions and cook gently until soft and translucent without browning

Pour in the water and add bay leaves thyme salt and black pepper then bring to a gentle boil

Lay the fish into the pot so it sits in a single layer and is barely covered by liquid

Cover and simmer on low heat until the fish flakes easily then remove carefully to a plate

Cool the fish briefly and gently flake into large pieces using your hands

Add clam juice and diced potatoes to the pot and simmer until the potatoes are fork tender

Heat the milk with the quartered onion layers in a separate saucepan and simmer gently to infuse flavor

Strain the milk into the pot and discard the onion solids

Return the flaked fish to the pot along with chives and a small pinch of nutmeg

Bring to a gentle simmer then turn off the heat

Slowly stir in the heavy cream taking care not to break the fish

Taste and adjust seasoning with additional salt and pepper if needed

Serve warm with optional butter chives and freshly ground black pepper on top