Bring a pot of salted water to a boil.
Add the spaghetti and cook until al dente, according to the package instructions.
Reserve about 1/4 cup of pasta cooking water, then drain the spaghetti.
In a skillet, cook the diced guanciale over medium heat until crispy and golden brown, about 5-7 minutes. Stir occasionally.
Add the tomato puree or passata to the skillet, along with the hot red pepper. Stir to combine.
Let the sauce simmer for 5-10 minutes, allowing it to thicken slightly. Season with salt to taste.
Add the cooked spaghetti to the skillet with the sauce.
Toss everything together, adding a splash of the reserved pasta water if needed to loosen the sauce and help it cling to the pasta.
Plate the spaghetti and sprinkle with grated Pecorino Romano cheese.