A combination of dry hard cider and dried seaweed brings a sweet, mild salinity to the stock, making it ideal for precooking the mussels and clams before using it as the base of the chowder.
Heat oil in a Dutch oven over medium. Add onion, carrots, and celery; cook, stirring occasionally, until softened and beginning to brown, 10 to 14 minutes. Add cider; cook, stirring occasionally, until liquid is reduced by half, 12 to 15 minutes. Add 7 cups water, kelp, thyme, garlic, and bay leaves. Bring to a boil over medium-high. Reduce heat to medium-low; simmer, uncovered, until flavors meld and liquid is reduced to about 4 1/2 cups, about 40 minutes.
Place mussels and clams in simmering stock; cover and cook until shells open, 8 to 15 minutes. Using a slotted spoon, transfer mussels and clams to a medium bowl; discard any that did not open. Let stand until cool enough to handle, about 5 minutes. Remove meat from shells; discard shells, and set meat aside. Pour stock through a fine wire-mesh strainer into a heatproof bowl; discard solids.
Wipe Dutch oven clean. Add butter, and melt over medium-high. Add onion; cook, stirring occasionally, until translucent, 6 to 8 minutes. Stir in potatoes, leeks, salt, and 41/2 cups strained stock (discard any remaining stock, or reserve for another use). Let mixture come to a boil. Reduce heat to medium-low; simmer, uncovered, stirring occasionally, until potatoes are tender, 20 to 25 minutes. Stir in cream; cook, undisturbed, just until chowder returns to a simmer. Add pollock; cook, stirring occasionally, until fish is almost cooked through, 2 to 3 minutes. Stir in reserved mussel and clam meat. Remove from heat. Let stand until mussels and clams are heated through and pollock flakes when pierced with a fork, about 5 minutes. Stir in dill; season with salt to taste. Ladle into serving bowls; garnish with ground nori and dill.
Stock can be prepared through step 1 and refrigerated 3 days in advance.
Find kelp or dried kombu on the international foods aisle at your local grocery store.