Cut all of the fish into 1½-inch cubes. Cover and refrigerate.

In a soup kettle, melt the butter over moderate heat. Add the onions and sauté until translucent, 3 to 5 minutes.

Add the heavy cream, milk, and thyme and reduce the heat to low. Simmer for 20 minutes.

Meanwhile, dice the fat back and cook over moderate heat until rendered golden brown. Drain on paper towels and set aside on a warm part of the stove. Add the potatoes to the chowder and simmer until the potatoes are almost fork-tender. Add the fish and simmer for about 10 minutes, until just cooked through. Season to taste with pepper.

Serve the rendered pork on the side and let each guest sprinkle some over the chowder.