How to Make It
Cook salt pork in a large Dutch oven over medium, stirring occasionally, until crisp, 10 to 12 minutes. Using a slotted spoon, transfer pork to a paper towel–lined plate; set aside. Wipe Dutch oven clean; discard drippings or reserve for another use.
Bring 3 cups water to a boil in a covered large, high-sided skillet over medium-high. Add clams; cook, covered, shaking skillet occasionally, until clams open, 10 to 12 minutes. Remove from heat. Using a slotted spoon or a spider, remove clams from cooking liquid. Discard any clams that did not open. Ladle 4 cups clam cooking liquid into a measuring cup; discard any sediment and additional liquid in skillet. If desired, reserve 30 clams in shell for garnish; cover and chill until ready to serve, up to 4 hours. Shuck remaining clams; discard shells. Chop clam meat into 1/3-inch pieces; set aside. (If not using the 30 clams in shell for garnish, shuck and chop all clams.)
Melt butter in Dutch oven over medium. Add onions and celery; cook, stirring occasionally, until softened, about 15 minutes. Stir in black pepper, thyme, and celery seeds.
Add potatoes and reserved 4 cups clam cooking liquid to onion mixture in Dutch oven. Bring to a boil over medium-high. Reduce heat to medium-low; simmer, uncovered, stirring occasionally, until potatoes are tender, 20 to 25 minutes. Using a slotted spoon, transfer 3 cups potatoes to a bowl (leaving cooking liquid in Dutch oven); mash using a potato masher until mostly smooth. Stir mashed potato mixture into remaining mixture in Dutch oven.
Stir half-and-half into mixture in Dutch oven; cook over medium-low, stirring occasionally, until just hot (do not boil), about 5 minutes. Stir in Finnan haddie, salt pork, and chopped clam meat; cook, stirring occasionally, until heated through, about 5 minutes. Season to taste with salt.
If garnishing bowls with reserved clams in shells, place 5 clams in each of 6 shallow serving bowls. Ladle about 2 cups chowder into each bowl. Garnish with parsley.
Finnan haddie may be purchased online at scottishgourmetusa.com.