Heat oven to 200C/fan 180C/gas 6. Brush the caps of the mushrooms with olive oil and heat a griddle (or frying pan).
Char the mushrooms on both sides until golden and softening, but not collapsing. Divide the breadcrumb mixture between 2 of the mushrooms, and put the other mushrooms on top, like sandwiches. Put in a roasting tin and bake for 5 minutes or until piping hot, while you get your burger fillings and sides ready. Open the buns and add them to the oven for another 5 minutes with the mushrooms.