Remove the bacon skin, chop it finely and saute in a pan without oil on a moderate stove. Add the julienned onions.
Wait for the bacon to redden nicely and for the onions to soften - about 7-8 min. Add the sour cream and stir for about 3-4 min. until it changes color from white to a nice yellow.
Add the yolks, beaten in advance, while stirring nonstop for about 2-3 min. until they coagulate. Sprinkle with a little ground black pepper.
In the meantime, boil the tagliatelle in a pot with water, salt and 2 tbsp olive oil, for the amount of time written on the package.