1. In fanon John Watson likes jam.
2. In Doctor Who 2x11 Fear Her we see the Doctor in someone's kitchen and without asking he just eats some jam. Right out of the jar. With his fingers.
3. We need a fic where the Doctor visits 221b for whatever reason, notices the jam and just helps himself.
4. PROFIT!!!

Alternative 3.: John is the fobwatched Doctor. I don't know why he didn't use Smith as his last name this time.