Just take a big portabello mushroom and marinate it however you'd like (and in accordance with your dietary restrictions) then grill it like you would a regular patty. Top with mustard, lettuce, tomato, onion, and then 'wrap' it in a big lettuce leaf.

OP here- I have to be gluten free. Low carb would be my preference, but not medically necessary. Gluten (especially the type in veggie burgers, causes serious issues).