In a heavy-bottomed saucepot over medium heat, render the salt pork until just crisp, 5 to 7 minutes. Remove with a slotted spoon and set aside. Sweat the onion in the pork fat until tender, 4 to 5 minutes. Add the potatoes and cover with milk. Bring to a boil, decrease the heat, and simmer until potatoes are soft, 10 to 12 minutes.
In a 3-quart saucier fitted with a steamer basket, bring the reserved clam juice to a boil. Add clams and cover. Steam until all clams are open, about 3 minutes. Remove the clams as they open and transfer to a bowl. Add the steaming juices to the chowder.
Using an immersion blender, puree to desired consistency. Season with salt and pepper, to taste.
Finely chop the drained canned clams. Fold into the chowder. Serve with steamed clams, crisp salt pork, parsley, sour cream, and grape tomatoes as garnish.