green salad leaves
While the burger is on the BBQ warm the bun either on top of the BBQ gril or in the toaster. Then put a layer of mayo on the bottom followed by some thinly cut green leaves. The top layer to spread on a thin layer of mustard.
Cookt he burger till it is starting to brown but not burned, as otherwise it will harden too much. Place on top of the leaves and put on the sliced tomato and the grated soya cheese. Then put the top bun on.