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1
Wash the clams well, scrubbing if necessary to remove external grit.
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2
(You can wash the clams hours or even a day in advance; keep them in a large bowl or colander set over a bowl, uncovered, in the refrigerator.)
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3
Put them in a pot with 1/2 cup of water and turn the heat to high.
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4
Steam, shaking the pot occasionally, until most of the clams are open, 7 to 10 minutes.
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5
Use a slotted spoon to transfer the clams to a broad bowl; reserve the cooking liquid.
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6
When the clams are cool enough to handle, shuck them over the bowl, catching every drop of their liquid; discard the shells.
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7
If any clams remain closed, use a thin-bladed knife to pry them open (it will be easy).
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8
Chop the clams.
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9
Strain all the liquid through a sieve lined with a paper towel or a couple of layers of cheesecloth.
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10
Measure the liquid and add enough water to make 3 1/2 cups.
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11
(You may prepare the dish in advance up to this point; refrigerate, covered, for up to a day before reheating.)
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12
Combine the liquid with the onion and potatoes in a saucepan; cover and bring to a boil.
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13
Reduce the heat to simmer, still covered, and cook until the potatoes are tender, about 10 minutes.
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14
Stir in the clams, season to taste with salt and pepper, and serve.
