Out Of Time - S1-E10
Continuity mistake: John is about to eat liver and potato with Emma before she storms out saying, "I don't like liver". While they are talking, the potato is facing Emma and Gwen. After Emma leaves, the liver faces Gwen and the spot where Emma was.
Answer: He might have requisitioned or stolen it from Torchwood London, or he might have been ready down below once it'd been severed, searching for "the right Doctor" and not knowing that the newly-regenerated Doctor was the one he was looking for.
Captain Defenestrator