Cook 1 pound pasta until al dente. While that's cooking, in a large pan on medium heat cook 1/2 pound diced pancetta and 1 cup chopped onions in olive oil until pancetta is browned and onions are soft.
Combine your cheese then pour half of it with your 4 egg yolks. Beat lightly until combined.
At this point your pasta and pancetta should be ready. Drain the pasta (reserve some of the pasta water) and transfer into your pan. Remove from heat.
Pour your egg mixture into the pan and toss the pasta until coated. Pour about 1/4 cup pasta water in your pan and continue tossing until you get the creaminess you want. If you need to put more pasta water, do so a little at a time so your pasta doesn't get too runny.
Add the rest of your cheese and toss some more. Season with salt and pepper as needed. Serve with chopped parsley.