Here's the one I use, taught to me by Rat Ba$tard Roberto from Brazil:
We like this one because it's simple and I always have these 4 ingredients and dried spaghetti. Also...this is a FAST meal...meaning you can't take that phone call from Great Aunt Agathea while making it. This dish deserves your full attention both while making and serving it up. Think Speedy!
1/2 pound of sliced bacon, cut across slices into 1" pieces.
1 small onion, diced.
3 eggs
1/2 -3/4 C freshly grated parmesan
** heavy cream, optional
spaghetti or linguini....you need a pasta that is long and thin so the eggs cook quickly.
Saute bacon and onion together until onions are soft and bacon has begun to crisp. This is critical timing....too soon and the bacon will be icky limpy; too long and the onions will burn. One fool-proof option is to saute the bacon until the fat renders and the bacon begins to crisp. Scoop bacon pieces from pan, but leave the fat. Add onions and saute until soft. Remove onions and add to bacon.
In a bowl, beat the 3 eggs until homogonized. Add the grated cheese. Set aside. If you want the dish to be super-rich, add 1/4 C heavy cream. I find this makes it taste more like al fredo than carbonara, so I leave it out.
Cook pasta in salted water until done to your liking. Strain the hot pasta water into the bowl you will be serving it in to warm it up. This is important too...otherwise, a cold bowl will start the fats re-congeling. Not a good thing. Put the strained pasta BACK into the hot pot it was cooked in--but OFF the heat--and dump int the bacon/onions. Stir to coat and then pour the egg/cheese mixture over the hot pasta mix. Take two forks and lift, lift, lift. The eggs immediately start cooking on the hot pasta and the cheese melts. If you don't move things around quick enough, the eggs follow their destiny and turn into scrambled eggs at the bottom of the pan instead of a silky creamy sauce. Not a good thing.
Scoop out 1 cup of the hot pasta water and dump out the rest. Check the texture of the coating...if you want it a bit thinner, dribble on a little hot pasta water. Grate lots of fresh pepper and add more parmesan. Pour into hot bowl. When serving, the bacon/onions tend to migrate to the bottom/sides of the bowl. Be sure to scoop those out.
(Thanks, Roberto. You were an a$$ in some ways, but you scored on this one.)