In a large heavy pot, melt the butter and olive oil over medium heat. Add the chopped onions, stir to coat in the oil and butter, and sauté until translucent and softened, about 5 minutes.

Add 2 cups of water, thyme, bay leaves, kosher salt, and black pepper to the pot. Bring to a boil.

Add the cod to the pot in a single layer. If needed, add more water until the fish is almost submerged. Bring to a low simmer, cover, and cook for 4-5 minutes until the fish flakes apart.

Remove the fish using a fish spatula and place on a plate to cool. Flake the fish into pieces.

Add the clam juice and diced potatoes to the pot. Cover and simmer on low for about 15 minutes, until the potatoes are tender.

Meanwhile, scald the milk with the quartered onion layers in a small saucepan. Bring to a boil, then simmer uncovered for 5-7 minutes.

Strain the scalded milk into the pot, discarding the solids. Add the flaked fish, minced chives, and nutmeg to the pot.

Bring the chowder back to a simmer. Turn off the heat and stir in the heavy cream gently.

Serve with optional garnish of butter, nutmeg, chives, and fresh ground black pepper.