In a saucepan, add the oil, guanciale, large onion petals, and garlic cloves. Heat on medium low until the guanciale has rendered out it’s fat and is golden and crispy. The garlic and onions should be softened and golden as well. Remove the garlic cloves and discard. Using a slotted spoon, remove the crispy guanciale and set aside as well.
Add the tomato paste and cook for 1-2 minutes to remove the raw flavour, then add the passata, salt and chili flake. Reduce until the “sauce’ is very dry (you will bring it together with the starchy pasta water! At this stage, you should almost be able to flip the pan upside down without the sauce base falling down!). Add the cheeses to the pan.
Cook the bucatini in boiling, salted water. When cooked, transfer the pasta to the pan. Add the starchy water and start to flip with the pasta, adding more water as needed to ensure that the sauce is coating the pasta without being too dry or too loose. Transfer the pasta to a serving bowl, then top with more cheese and a sprinkling of chives.
