TRIM OFF ANY TOUGH OUTER SKIN FROM A PIECE OF GUANCIALE. CUT INTO 1-INCH BATONS AND BROWN LIGHTLY IN A CAST IRON PAN WITH A SPLASH OF OLIVE OIL ON A LOW FLAME.
MEANWHILE, PEEL AND TRIM ONE LARGE RED ONION. SLICE LENGTHWISE IN HALF. SLICE CROSSWISE INTO 1/4-INCH HALF MOONS.
WHEN THE GUANCIALE IS BARELY BROWNED, STIR IN THE ONIONS. SALT LIGHTLY. PEPPER GENEROUSLY. ADD A PINCH OF CHILI FLAKES (1/8 TSPS - 1/2TSP, DEPENDING ON HOW HOT YOU LIKE IT.).
BRING A POT ON WATER TO A BOIL. SALT GENEROUSLY, THEN REDUCE TO A SIMMER TILL YOUR READY TO ADD THE PASTA.
COOK THE ONIONS, STIRRING AND TOSSING OCCASIONALLY FOR ABOUT 15 MINUTES UNTIL THEY ARE SLIGHTLY CARAMELIZED.
MEANWHILE, SCORE THE SMOOTH TOP OF THE TOMATOES IN A 1-INCH CROSS-HATCH WITH A SERRATED KNIFE.
PLACE A BOX GRATER IN A BOWL. GRIPPING THE TOMATOES ON THE STEM END, GRATE THEM THROUGH THE LARGEST HOLES ON THE GRATER, SO ONLY THE PULP WINDS UP IN THE BOWL.
STIR THE TOMATO PULP INTO THE ONIONS. COOK ON A VERY LOW FLAME TILL THE PASTA IS DONE, BUT DON'T LET ALL THE LIQUID BOIL AWAY.
COOK THE SPAGHETTI, SCOOPING OUT A CUP OF PASTA WATER TOWARDS THE END OF THE COOKING.
DRAIN THE PASTA. RETURN TO THE POT ALONG WITH THE ONION SAUCE. ADD ABOUT A 1/2 CUP OF THE PASTA WATER AND COOK ON A MEDIUM LOW FLAME TILL THE SPAGHETTI ACHIEVES THE CONSISTENCY YOU LIKE.
SERVE IMMEDIATELY WITH FRESHLY-GRATED PARMESAN CHEESE AND/OR PECORINO ROMANO.
SERVES 4
