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.