A Roman soup of pureed salt cod and potatoes, this dish belongs to the fall and winter seasons when fresh fish is harder to come by and potatoes were traditionally one of the few available fresh vegetables. Versions of this soup are found in many cold-weather areas, including Norway and Finland, where it’s made with “stockfish,” which is dried but unsalted cod or other fish. The addition of a little lemon zest is actually quite important, because its oils lift the character of the soup, keeping it from being too heavy or murky; for a subtler lemon taste, blanch the zest for 10 seconds in boiling water. Note that the salt cod must be soaked for 36 to 48 hours; for more about salt cod.
Ingredients
- 2 pounds salt cod
- 1 large Spanish onion, halved through the root end, 1 half left
- intact, the other coarsely chopped
- 2 celery stalks, trimmed, 1 halved crosswise, 1 coarsely chopped
- 2 bay leaves, preferably fresh
- ¼ cup olive oil
- 4 garlic cloves, smashed with the side of a chef’s knife and peeled
- 2 cups thinly sliced leeks (white and light green parts only)
- Kosher salt
- Freshly ground black pepper
- 2 pounds Yukon Gold potatoes, peeled and cut into small dice
- Pinch of finely grated lemon zest
How to Make It
- Put the salt cod in a medium bowl and cover by at least 1 inch with cold water. Cover loosely with plastic wrap and refrigerate for at least 36 hours, or up to 48 hours, draining the water and replacing it with fresh, cold water every 8 hours or so. The best way to know if you’ve soaked the fish long enough is to stick a clean index finger into the water and taste it; you want a hint of salt but not an excessively salty flavor.
- Put the salt cod in a medium heavy pot and cover by 1 inch with cold water. Add the onion half, halved celery stalk, and bay leaves and bring to a simmer over medium-high heat. Lower the heat and simmer until the fish can be flaked, 4 to 5 minutes. Use tongs or a slotted spoon to transfer the salt cod to a plate. When it is cool enough to handle, remove any skin or bones and discard them. Set the salt cod aside.
- Heat a wide, deep, heavy pot over medium-high heat. Pour in the oil and tilt to coat the pan; heat the oil until it is shimmering and almost smoking. Add the chopped onions, chopped celery, garlic, and leeks, season with salt and pepper, and cook, stirring with a wooden spoon, until the vegetables are softened but not at all browned, about 5 minutes.
- Stir in the potatoes, season with salt and pepper, and pour in enough cold water just to cover them. Bring to a simmer and cook until the potatoes are tender to the tines of a fork, about 8 minutes. Remove from the heat.
- Working in batches, transfer the contents of the pot and the salt cod to the bowl of a food processor and pulse just until uniformly smooth; transfer to a bowl. If the soup seems too thick, whisk in a few tablespoons of water.
- Rinse out the pot, add the soup, and reheat over medium-high heat. Stir in the lemon zest. Taste and adjust the seasoning with salt and pepper. Divide the soup among 6 bowls and serve.
- Variations This soup lends itself to a number of personal adjustments: To enrich it, make it with fish or chicken stock in place of the water, or with a mixture of cream and water or stock.
- For a more brothy soup, puree only half of it. For a more modern, elegant variation, don’t puree any of the soup. Serve the broth, floating flakes of the salt cod on the surface.
- Finish the soup with a drizzle of Salsa Verde or colatura.
