I've already posted a few versions of mini burgers, such as here and here, but this one might be my best yet, and I discovered it entirely by mistake. On Christmas Eve, we made Chloe Coscarelli's stuffed portobellos and were left with four of them (ten shrooms; six guests). You can, of course, make this from scratch, but I'd recommend this as the next-day meal after you make the mushrooms.
The only additional ingredient you need is about a half-cup of seitan. Place the stuffed mushrooms--stuffing, tomato, and all--in the food processor with some seitan and process until smooth. Make two-inch burgers and grill them with some vegetables. I can totally see taking the mix to a picnic, in a tupperware, and forming and grilling the burgers in situ.