Not really a 'sandwich'
but Dr. Laffa will definitively fill you up.
Might be a little north for you, but the prices are cheap for the quantity you get. A falafel/shawarma laffa (never get pita) is enough for 2 massive meals, and they will cut it in half for you if you are sharing. I tend to eat half for lunch and save the rest for dinner/midnight snack.