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Heat the olive oil in a large sauté pan over medium high heat. When the pan is very hot, add the onions and salt and combine well. Let the onions sit in the hot pan until they are starting to sweat. There will be a lot of liquid initially as the onions cook. Stir them after 5-7 minutes and let them just cook for another 5 minutes without stirring. Continue this process until most of the liquid is gone. Be patient as this will take awhile.
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Once the onions are soft, turn the heat up and, stirring constantly, brown the onions. As the onions brown and get dry, add the wine, one tablespoon at a time. Remove the onions from the pan and set aside.
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Bring a large pot of salted water to a boil.
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Add the bacon to the empty pan and over medium heat and cook until crispy. Remove the bacon from the pan and drain on paper towels, discard the grease and wipe the pan dry. Put the caramelized onions in the pan (no oil is necessary) and over a medium heat add the garlic and cook for two minutes. Add the drained bacon to the onions and the prosciutto. Stir for 2 minutes until the prosciutto is just warm. Add the chicken broth to the pan and heat the mixture until it just starts to boil. Keep warm over a very low heat.
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In a small bowl, beat the egg yolks, cheese and cream (if you are using it).
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Cook the pasta according to the package directions. You want the pasta to be al dente but not overly so.
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When the pasta is cooked, remove ½ cup of pasta water and set it aside. Drain the pasta quickly. Transfer the onion and bacon mixture from the sauté pan to the pasta pot and remove the pot from the heat. Add the pasta all at once to the onions and quickly add the egg and cheese mixture. Toss the pasta very quickly off of the heat with a large fork and spoon until all of the ingredients are mixed together. Add black pepper to taste.
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Add the hot pasta water at the end and toss quickly until the water coats the pasta and the ingredients are all mixed together.
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Toss on the parsley and serve with additional cheese on the side.