How to Make It
In a large pot, bring 1/2 inch of water to a boil. Add the clams, cover and steam over moderately high heat until just opened, 5 to 7 minutes. Remove the pot from the heat. Using a slotted spoon, transfer the clams to a rimmed baking sheet; discard any that don’t open.
Strain the cooking liquid through a cheesecloth-lined sieve into a large bowl. Wipe out the pot and return the cooking liquid to it. Add enough water to the pot until three-fourths full and bring to a boil. Generously salt the water, add the spaghetti and cook until al dente. Drain, reserving 1 cup of the pasta water.
Meanwhile, remove the clams from their shells; discard the shells. Thickly slice the clams. In a small bowl, whisk the egg with the egg yolks, the 3/4 cup of cheese and the nutmeg.
Heat a large skillet over moderate heat. Add the pancetta and cook, stirring occasionally, until lightly browned and most of the fat is rendered, about 10 minutes. Using a slotted spoon, transfer the pancetta to a plate; leave the fat in the skillet.
Add the onion and garlic to the skillet and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Add the pancetta, clams and spaghetti and stir to coat, about 1 minute. Remove the skillet from the heat and slowly drizzle in the egg mixture, tossing, until incorporated. Add the reserved pasta water, the chopped parsley and butter and toss to coat; season with salt and pepper. Divide the pasta into bowls and serve, passing more cheese at the table.