1. In a 5-quart saucepan, melt butter over medium-high heat. Add scallions, celery, and garlic; cook until tender, 2 to 3 minutes. Add flour; stir to coat. Add clam juice and 2 cups water, whisking until smooth.

2. Add potatoes, thyme, and bay leaf. Bring to a boil; reduce to a simmer. Cook until potatoes are tender when pierced with the tip of a paring knife, about 10 minutes.

3. Add cod; cover, and cook until fish easily flakes, about 8 minutes. Pour in milk; cook until heated through, 1 to 2 minutes. Remove and discard bay leaf. Season with salt and pepper. Serve immediately.