Saute 4 onions in 4 tbsps butter.
Transfer sauteed onions to a blender or food processor.
Add 1/2 cup olive oil, 3 egg yolks, 3/4 cup parmesan cheese and salt and pepper to taste. Blend until creamy.
Transfer sauce back to pan on low heat.
Cook 1 lb spaghetti or other pasta. When pasta is cooked al dente, transfer pasta into sauce pan using tongs. DO NOT DRAIN.
Add a splash of pasta water to the pan to thicken sauce. Mix well.