In a large skillet (big enough to hold the pasta later), lightly sauté the prosciutto cut into strips with a tablespoon of olive oil. When it starts to brown, add a tablespoon of water to stop the cooking.

In a separate small pan, melt the butter with a scant ladle of water and the finely chopped onion or leek. Cook gently until very soft and aromatic, but not browned. At the end, add this mixture to the skillet with the sautéed prosciutto.

In a bowl, beat the eggs and add the lemon juice, then mix in the Parmesan and heavy cream until well combined.

Cook the fettuccine and drain when al dente. Add them directly to the skillet with the onion and prosciutto over medium heat, tossing for a few minutes.

Turn off the heat. Pour the egg, cream, Parmesan, and pepper mixture into the skillet, and quickly toss everything together so the eggs thicken slightly and the fettuccine stay creamy and don't clump. If the pasta seems too dry, add 2 to 3 tablespoons of milk.

Serve immediately while hot.
Note: The lemon juice helps remove any eggy smell and adds aroma, making the pasta more flavorful.