I tried cabbage leaves and collard greens, both raw and cooked, and lightly microwaved collards emerged as the clear winner. Here are the details:
Cabbage leaves seemed like a promising bun alternative, since they already have a bit of curve to them and are a good size, but I found them a little too crunchy and varied in texture. Raw, they broke easily and let delicious toppings spill out. I hoped some cooking in a microwave would soften them and solve this problem, but it would have taken a whole lot of heat to soften the thick, backbone part of the leaf enough to keep it from being an excessively crunchy distraction. Removing that part of the leaf didn’t really leave me with enough intact material to adequately wrap the burger…so I gave up on cabbage for this purpose.
The testing I did with a few nice, large collard greens was much more successful. I removed the lowest, thickest part of the stem, but was still left with plenty to work with. The leaves were uniformly flexible, both raw and cooked, and their flat, uniform shape made them easy to shape into an effective wrap for both burger and toppings. I did prefer the cooked version (again, I microwaved, but I’m sure steaming would work, too), but raw was fine, too.
We’ll have to do some more experimenting on cooking technique, and perhaps do some more testing on toppings, too, but I’ve got my vegetable bun picked out for the next round.