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Method

1. Peel and slice the potatoes about 1/8-inch thick. Peel and dice the onion fine. Clean the fish, trim the fins with scissors, remove the head, and gut the fish. Cut each of the cleaned fish into three thick slices.

2. In a 3-quart soup pot, layer the potatoes and the onion alternately until both are used up. Season each layer of potatoes with salt and pepper. Pour the olive oil over the potatoes and add water until the potatoes are barely covered. Season the fish and lay the fish on top of the potatoes in a single layer. Cover the pot and bring it to a boil over high heat. Shake the pot occasionally to prevent the potatoes from sticking to the bottom and scorching. Once the pot boils, reduce the heat slightly but maintain a strong boil.

3. After about 5 minutes begin checking the fish for doneness—the tail sections will cook faster than the thicker sections from near the gills. When the fish is barely cooked, remove it to a platter and return the cover to the pot and continue to boil. The action of the boiling along with the gelatin from the skin of the fish will make the olive oil emulsify. Check the bottom of the pot periodically to be sure the potatoes aren't catching or scorching. Add additional water if the soup is getting too thick.

4. Once the fish is cool enough to handle, pick the meat from the bones and discard the bones and the skin. Flake the fish into bite-sized pieces and reserve along with any liquid that has gathered on the platter with the fish.

5. After about 20 minutes of boiling, the potato and onion should be tender. Add the lemon juice to the soup and pass it through a fine food mill (it should be nearly puréed already). Adjust the seasoning with salt, pepper, and lemon juice if needed.

6. Reheat the soup after you are happy with the flavor. Once hot, add the fish and the parsley and serve immediately in warm soup plates before the fish over cooks.