"I haven't seen you in ages, how about I come to [your town] and take you out to dinner?"

Harder for you given the limitations in what you can eat though...

Indeed :-/ Several people, here and elsewhere, have suggested restaurants as a means of getting round the issue, but they unfortunately aren't really feasible for me.

offer to take with you a casserole dish containing a Delicious Meal which they can shove in the oven ...

I'm with Lnr on this one. IF you can find a suitable place.

And the whole point is to leave your friend in no doubt that you miss them and want to see them, however you manage to sort it. Yes it will warent a discussion, but that is ok.