2 tablespoons unsalted butter
2 medium onions, very thinly sliced
4 stalks celery, very thinly sliced
2 medium carrots, very thinly sliced
2 dried bay leaves
1/4 cup roughly chopped fresh flat-leaf parsley leaves and stems
6 to 8 sprigs fresh thyme
2 tablespoons black peppercorns
1-6" or 2-4" large fish heads from cod or haddock, split lengthwise, gills removed, and rinsed clean of any blood
2 1/2 to 3 pounds fish bones (skeletons) from sole, flounder, bass, and/or halibut, cut into 2" pieces and rinsed clean
1/4 cup dry white wine
About 2 quarts very hot or boiling water
Kosher or sea salt
Melt the butter in a heavy 7- to 8-quart stockpot over medium heat, add first 8 ingredeints, stirring frequently with a wooden spoon, until the vegetables become very soft without browning (7-10 8 minutes).
Place the fish head on the vegetables and stack the fish bones evenly on top. Pour in the wine, cover the pot tightly, and let the bones sweat until they have turned completely white (10-15 minutes. Add enough very hot or boiling water to just barely cover the bonesstir gently, and bring to s simmer for 10 minutes, uncovered (skim foam as you go).
Remove the pot from the stove, stir and allow it to steep for 10 minutes, then strain through a fine. Use immediately or refrigerate and use within 3 days - can be frozen for 6-8 weeks. Makes about 2 quarts.