SLICE THE GUANCIALE 1/4" THICK THEN INTO PIECES ABOUT 1/2" SQUARE. PLACE IN A CAST IRON PAN WITH JUST A SPLASH OF OLIVE OIL.
COOK ON A LOW FLAME UNTIL THE GUANCIALE IS BROWNED BUT STILL A BIT SOFT.
MEANWHILE, PEEL AND SLICE THE ONION 1/4" THICK. YOU SHOULD HAVE AN ALARMING PILE OF ONIONS. DON'T WORRY, THEY COOK DOWN A LOT.
WHEN THE ONIONS HAVE BROWNED ADD THE ONIONS. SPRINKLE WITH A LITTLE SALT, PEPPER AND CHILI FLAKES IF YOU LIKE. COVER. COOK ON LOW FLAME TURNING WITH TONGS EVERY TEN MINUTES OR SO. IT WILL TAKE AT LEAST 20 MINUTES FOR THE ONIONS TO START TO CARAMELIZE.
UNCOVER AND CONTINUE COOKING AND TURNING THE ONIONS TILL THEY ARE A DARK GOLD IN COLOR.
BRING A LARGE POT OF WATER TO A BOIL. SALT GENEROUSLY. BOIL THE PASTA AS DIRECTED (ABOUT 10 MINUTES). TOWARDS THE END OF THE COOKING, SCOOP OUT A CUP OF PASTA WATER AND RESERVE.
DRAIN THE PASTA, THEN RETURN IT TO THE POT. PLACE OVER A MEDIUM FLAME. ADD THE GUANCIALE AND ONIONS, 1/2 CUP OF PASTA WATER AND 1/2 CUP GRATED CHEESES.
STIR VIGOROUSLY WITH A WOODEN SPOON UNTIL MOST OF THE LIQUID HAS EVAPORATED AND THE PASTA HAS A CREAMY TEXTURE.
EMPTY INTO A WARM PASTA BOWL*. SCATTER WITH ROUGHLY-CHOPPED PARSLEY. SERVE WTIH MORE CHEESE ON THE SIDE.
SERVES 4
*NOTE: PLACE THE EMPTY PASTA BOWL IN THE SINK UNDER THE COLANDER. THIS WAY, WHEN YOU DRAIN THE PASTA, THE HOT WATER WILL WARM THE BOWL.