Garlic or onion, pecorino or parmesan, bacon or ham, cream or butter – how do you like your carbonara, and what's the secret to getting that perfect consistency? Do you, blasphemously, add peas, as they tend to in the States, or mushrooms, like my dad? And can anyone tell me why you never see carbonara's close relative, fettucine alfredo, in the UK when it's so madly popular in America?