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.