Step 1

Melt butter in Dutch oven over medium heat. Add onions, salt pork, thyme, ¾ teaspoon salt, and bay leaf; cook, stirring occasionally, until onions are softened but not browned, 3 to 5 minutes. Add water and bring to simmer. Remove pot from heat, gently place cod fillets in water, cover, and let fish stand until opaque and nearly cooked through, about 5 minutes. Using metal spatula, transfer cod to bowl.

Step 2

Return pot to medium-high heat, add potatoes, and bring to simmer. Cook until potatoes are tender and beginning to break apart, about 20 minutes.

Step 3

Meanwhile, whisk milk, cornstarch, and ½ teaspoon pepper together in bowl. Stir milk mixture into chowder and return to simmer. Return fish and any accumulated juices to pot. Remove pot from heat, cover, and let stand for 5 minutes. Remove and discard salt pork and bay leaf. Stir gently with wooden spoon to break fish into large pieces. Season with salt and pepper to taste. Serve immediately.