Forty five minutes into the stock-making time, combine leeks, onions, butter, and salt in a large soup pot, and begin sautéing on medium-low. Wash 1.5 lb clams, place in a colander, and set in the steam from the stock pot, with the lid on top of the colander; steam the clams 15 minutes while the alliums cook, so that the clams completely open. Meanwhile, wash and dice a pound or so of potatoes. Sprinkle potatoes with lemon juice (half a lemon; use the other half for salad dressing) to prevent discoloration if the potatoes are to sit out a while.
When the clams open, remove them from the heat. Set a sieve over the soup pot with the leeks, and carefully ladle in stock, regularly dumping out the fish, crab, etc., into the trash. Fill the pot 3/4 full. (Transfer any remaining stock, strained, into freezer-safe mason jars (wide-mouth pints), let cool to shower temperature, and label and freeze.)
Add potatoes to the soup and bring back to a boil. Cook 10-15 minutes, checking that the potatoes are cooked through but not falling apart. Meanwhile, shell the clams with a small spoon. Turn off the heat, add the clams to the soup, and add one cup cream and some salt. If you have any chives or parsley butter, add it at the end too.
Serve hot with whole-wheat bread or a good sourdough. Have a very chilled wine and copious icewater — soup warms you up quickly.