2 pounds razor clams, scrubbed (littleneck or manila clams can be substituted) |
Fill a medium saucepan with a ½-inch depth of water. Bring the water to a boil. When the water is boiling, add the clams, cover, and steam until the shells open, about 8 minutes. Remove from the heat and use a slotted spoon to remove the clams from the pot, reserving the liquid in the pot. Discard any clams that do not open. Remove the clams from their shells, discard the shells, and coarsely chop the meat into pieces about the size of a nickel. Set aside. Strain the clam liquid through a fine-mesh strainer into a large measuring cup.
Bring a large pot of salted water to a boil and add the strained clam liquid. Add the spaghetti and boil until al dente. Drain, reserving 1 cup of the cooking water.
While the spaghetti cooks, heat a 12-inch skillet over medium heat. Add the salt pork and sauté for about 10 minutes, until the fat has rendered and the pieces of salt pork are browned and crisp. Remove from the pan with a slotted spoon and let drain on a paper towel–lined plate. Add the onion and garlic to the pan and sauté for 5 minutes, until the onion is soft. Add the chopped clams and sauté for 2 minutes more, then add the spaghetti and toss until the spaghetti is warmed through and the mixture is well combined.
In a small bowl, whisk together the whole egg, egg yolks, Parmigiano-Reggiano, and nutmeg. Remove the spaghetti from the heat and carefully drizzle in the egg mixture, tossing the noodles as you go to prevent the egg from scrambling, until fully incorporated. Add a splash of the reserved pasta cooking water and toss until the pasta looks silky. Season with salt and a generous amount of pepper, then stir in the butter, parsley, and lovage, if using. Transfer to a platter or serving plates and serve warm.