In a heavy-bottomed saucepot, render the salt pork until just crisp. Remove and discard. Sweat the onion in the pork fat until tender. Add the potatoes and cover with milk. Bring to a boil and let simmer until potatoes are soft. Season with salt and pepper.
In a separate pot, fitted with a steamer basket, bring the reserved clam juice to a boil. Add clams and cover. Steam for three minutes until all clams are open. Remove the clams as they open and transfer to a bowl. Add the steaming juices to the pot with the chowder. Using a stick blender puree to desired consistency. Adjust seasonings.
Finely chop the drained canned clams. Fold into the chowder. Serve with steamed clams, parsley, sour cream and grape tomatoes, as garnish.
Recipe courtesy of Alton Brown