Bring a large pot of salted water to boil.

Cook the bacon or pancetta over medium heat in a skillet large enough to hold all your pasta once it's cooked. Cook, stirring, about 5 minutes.

Add the onion to the pancetta. And now is a good time to add your spaghetti to the boiling water and set your timer for however long the pasta takes. Mine was 11 minutes.

Cook the onion and pancetta another 5 minutes, keeping them in separate halves of the pan, stirring. After the 5 minutes, stir them altogether.

Add 2 cups of the pasta water to the onion and pancetta. Bring to a boil and keep at a boil, stirring now and then. Cook this down to about half.

In a small bowl, beat the egg slightly then add 1/8 cup hot pasta water and stir into the egg. (This keeps the egg from scrambling when you add it to the pasta.)

When the sauce has reduced by half and the spaghetti is cooked al dente, scoop the pasta up with tongs, let it drain a little and then put it into the skillet with the pancetta and onion. Stir in the scallions , pepper and salt to taste.

Toss the pasta until the scallions are wilted. Turn off the heat.

Quickly mix in the egg, stirring until the spaghetti is coated and creamy. Mix in the cheese and serve.