When did grilled-vegetable sandwiches become ubiquitous? In my (admittedly faulty) memory, they just popped out of nowhere, full-grown and popular, like Serge Prokofiev's Lieutenant Kije.
I suspect the sandwiches were for vegetarians dining in the mainstream. Early models rarely arrived without goat cheese and grilled eggplant, red peppers, and zucchini. But the real talent inside the bun was the big, fat, juicy portabella. This mushroom grills so quickly and is so meaty that a portabella burger now appeals to me as much as a good hamburger.
The secret to a super-juicy portabella burger is gravity. Start grilling the cap with the gill side down until the juices begin to seep. Then turn it over so the flavorful liquid can accumulate. Drop some roasted peppers into the cup, cap it with cheese, and slide the "burger" into a toasted bun. With salad, a glass of Merlot, and the slanting rays of the setting summer sun, dinner's ready.