Just use mashed potatoes, vegetarian gravy, lots of mixed
   veggies. You don't even need the meat in it. if you want to
   substitute, you could use the various tofu crumbles, "wheat
   meat" crumbles, get some basic boca burgers, cook it up and
   smash it into small pieces and put it in there instead. Veggie
   sausage would work too, or you could try using cooked lentils
   for the meat portion. The beauty of shephards pie is that its
   really simple, and there's no real wrong way to do it. "