For the onions:

1. Cut both ends off the onions then cut in half lengthways, peel and then slice in half circles 1/4 inch thick.
2. Melt the 2 tablespoons of butter in a heavy bottomed skillet or dutch oven over a medium high heat and cook the onions for 10 minutes until softened. Stir them so they don't catch.
3. Season with a good couple of pinches of sea salt and then turn the heat down to low and continue cooking for 30 minutes (it might take longer depending on your stove top) or until golden and super soft. You'll want to stir every couple of minutes, they mustn't burn so try and have the heat as low as possible.
4. Add the balsamic, mix well and continue cooking for 5 minutes then set aside.

For the sauce

1. Get a pot of well salted water boiling and depending on whether you are using fresh or dried pasta will dictate when you add the pasta to the water. The sauce only takes a few minutes to bring together so when using fresh pasta I add the pasta to the water when the butter is just starting to melt.
2. Melt your butter in a large skillet over medium - high heat and add the pepper.
3. Let it sizzle together for a minute then turn down the heat to low.
4. Add 1/2 cup pasta water and whisk vigorously to emulsify. I like to shake the pan with my left hand and whisk with my right to agitate it quite vigorously. It should be simmering very gently.
5. Keep agitating the pan and whisking until it thickens slightly 1 - 2 minutes.
6. Add your cooked pasta into the pan, toss with the liquid and let it bubble away gently for a minute or so.
7. Arrange the pasta into an even layer in the pan and then sprinkle the cheese over top in an even layer over top.
8. Leave it alone and let it melt gently on top for a minute or so, the heat should be nice and low so the pasta doesn't stick to the pan. Give the pan a shake if you are worried it is sticking.
9. Then with tongs toss the pasta over and over quite quickly until a silky sauce appears. 
10. Add the onions in and give it a quick toss then serve immediately.