Drain clams, reserving juice into a measuring cup. Roughly chop clams. Set aside.
To the clam juice, add enough water to equal 1 cup of liquid. Pour into a large bowl, along with the stock (or to additional clam juice/water, whichever you are using.) Set aside.
Scrub potatoes (or peel, if you prefer) and cut into 1/2" cubes. Place into the bowl with the stock & clam juice mixture to keep them from oxidizing.
In a large pot or dutch oven, fry bacon lardons until just crispy. Remove to a paper towel lined plate, leaving bacon grease in pot. You should have about 3-4 tablespoons of bacon grease. If you do not, add 1 tablespoon of butter.
Add onions & celery to the pot and sauté over medium heat until they start to soften, about 5 minutes. Add garlic and celery seed and cook another minute.
Add 3 tablespoons of flour. Stir and cook for another 2-3 minutes.
Add the potatoes and broth, as well as the half and half. Simmer on the stove until potatoes are fork tender (about 20-30 minutes).
Right before serving, add the chopped clams and bacon (reserving a little bacon for garnish, if desired.) Cook another couple minutes, until clams are warmed through.
Ladle into bowls and top with some of the reserved bacon, scallions and celery leaves.