You think that you need double cream...NO!
You think that you need onions...NO!
You thinking that you need to cook eggs...NO!
The misinformation about this Roman dish is huge. I'll try to give you the authentich recipe.
320 gr spaghetti (serving 4)
130 gr cured pork cheek (guanciale) or bacon
3 egg yolks
black pepper to grind coarsley
pecorino romano cheese
Set a skillet over medium heat when hot put the cured pork cheese or bacon in cubes to brown then lay on paper towel to absorbe the fat.
In a glass or ceramic bowl (not in metal otherwise th eggs would cook) put the yolks with the pepper and the pecorino romano cheese and mix grossly and quickly (You must not whip the eggs since they must remain creamy and not fluid).
Drain very well the pasta (no water at all) and fold it into the egg cream and mix powerfully, adding the bacon or guanciale. Do not add salt since the bacon or guanciale and the pecorino cheese are naturally salty.
Serve with other percorino cheese and black pepper.