3 tablespoons unsalted butter
2 medium onions (14 ounces), cut into 3/4-inch dice
6 to 8 sprigs fresh thyme, leaves removed and chopped (1 tablespoon)
1 – 2 dried bay leaves
2 pounds Yukon Gold, Maine, PEI, or other all-purpose potatoes, peeled and sliced 1/3-inch thick
5 cups Fish Stock, Chicken Stock, or water (as a last resort)
Sea salt and freshly ground black pepper
3 pounds skinless haddock or cod fillets, preferably over 1 inch thick, pinbones removed
1 1/2 cups heavy cream (or up to 2 cups if desired)
2 tablespoons chopped fresh Italian parsley
2 tablespoons minced fresh chives
Heat a 4- to 6-quart heavy pot over low heat and add the butter, onions, thyme, and bay leaves to the pot. Sauté, stirring occasionally with a wooden spoon, for about 8 minutes, until the onions and softened but not browned.
Add the potatoes and stock. If the stock does not cover the potatoes, add just enough water to cover them. Bring to a boil, cover, and cook the potatoes until they are soft on the outside but still firm in the center, about ten minutes. Reduce the heat to low and season with salt and pepper (you want to almost over-season the chowder at this point to avoid having to stir it much once the fish is added). Add the fish fillets and cook over low heat for 5 minutes, then remove the pot from the heat and allow the chowder to sit for 10 minutes (the fish will finish cooking during this time).
Gently stir in the cream and taste for salt and pepper. If you are not serving the chowder within the hour, let it cool a bit, then refrigerate; cover the chowder after it has chilled completely. Otherwise, let it sit for up to an hour at room temperature, allowing the flavors to meld.
When ready to serve, reheat the chowder over low heat: don’t let it boil! Ladel into bowls and sprinkle with chopped parsley or minced chives.