The rest was really simple, just brushing sliced eggplant (from a local greenhouse) and zucchini with oil, seasoning them with salt and pepper, and grilling them until they were soft and had nice grill marks on them (I had to use a grill pan because the resident barbeque is broken). Then I halved the buns, layed them out on a baking sheet with the tops beside them, and piled the grilled veggies on top, finishing each one off with a slice of Swiss cheese from Vancouver Island. I then placed the baking sheet under the broiler just long enough to melt the cheese, and placed the top of the buns back on just before serving.
The grilled veggies were delicious, and the mushrooms were meaty and juicy. I think I will be using portobello mushroom caps instead of store bought "veggie burgers" more often, and maybe I'll experiment with different kinds of cheeses (chevre or smoked gouda perhaps?).
What a fun way to eat your veggies!