Cook the spaghetti al dente according to the instructions on the packet.
In a bowl, stir together the cream, 3 egg yolks, parmesan and finely chopped parsley. Season with salt and pepper.
In a large saucepan, sauté the onions and bacon with a little butter. Add the spaghetti and stir vigorously.
Turn the stove down and add the cream sauce. Let the sauce thicken while stirring constantly. If necessary, turn the stove up a little - but make sure that the sauce does not stagnate too much.
Arrange on plates and serve garnished with a basil leaf or the remaining parsley.
Tip: I always add a little dried and grated chilli to the cream sauce, then the whole thing gets a little kick.