I thought the same thing about the inside of the pie, but apparently it was to show he had killed a dove that was inside (or so I'm told).

My guess is the Tyrells with the pie. Margaery did tell 'ol Joff to have a second slice while she had none. And I'm not sure we saw anybody else eat it, though I could be wrong about that bit. He'd been drinking wine all day, the pie was the new ingredient.