Step 1

Melt 1 Tbsp. butter in a large heavy pot over medium heat. Cook onion and celery, stirring often, until translucent, about 5 minutes. Mix in Old Bay and 1 tsp. pepper; cook, stirring, 30 seconds. Add sherry; cook, stirring occasionally, until alcohol has cooked off, about 1 minute. Add ½ cup clam juice; bring to a simmer. Cook, stirring occasionally, until vegetables are very soft, about 5 minutes. Add stock and remaining 2 cups clam juice and bring to a boil. Add potatoes and simmer, uncovered, adjusting heat as needed, until just tender, about 5 minutes. Add fish and shellfish and cook, stirring gently, just until mixture returns to a simmer, about 2 minutes. Remove from heat and stir in lobster meat if using.

Step 2

While the potatoes are simmering, melt remaining 1 Tbsp. butter in a small saucepan over medium heat. Whisk in flour and cook, whisking constantly, until light blond in color, about 1 minute. Whisk in half-and-half and 1 cup chowder broth (skimmed off the top) and bring to a simmer, whisking constantly. Cook, still whisking, until thickened and creamy.

Step 3

Stir half-and-half mixture into chowder ; return to medium heat. Bring to a simmer, stirring gently. Remove from heat; season with salt and more pepper. Ladle into bowls and serve with oyster crackers.