1/2 cup freshly grated Pecorino Romano cheese, plus more for serving
Kosher salt and freshly ground pepper
In a large, deep skillet, heat the olive oil until shimmering. Add the guanciale and cook over moderate heat until browned and slightly crisp, about 7 minutes. Using a slotted spoon, transfer the guanciale to a plate. Add the onion, garlic and crushed red pepper to the skillet and cook over moderately low heat until the onion is golden brown, about 5 minutes. Return the guanciale to the skillet and keep warm.
Meanwhile, in a large pot of salted boiling water, cook the spaghetti until al dente. Drain the pasta, reserving 1/2 cup of the cooking water. Add the pasta to the skillet and cook over moderately high heat for 1 minute. Add the reserved cooking water and the 1/2 cup of grated cheese and toss to thoroughly coat the spaghetti. Season with salt and pepper. Transfer to shallow bowls and serve immediately, passing additional Pecorino Romano at the table.