Use a heavy-bottomed skillet to distribute heat evenly, which prevents the onions from burning during the long caramelization process. Deglaze the pan with a small splash of pasta water if the bottom becomes too dark to ensure all the fond ends up in your sauce. Cook your pasta until it is just shy of al dente, as it will continue to soften once it hits the sauce in the pan. Grate your own parmesan cheese from a block rather than using pre-grated versions to achieve a smoother, creamier texture in the final sauce.