In a bowl combine the lamb, onion, garlic, cumin, breadcrumbs, parsley and half the mint, season and mix well. Form into 4 patties. Press 1 mushroom, stalk-side up into the centre of each pattie and re-shape. Place onto a tray, cover and refrigerate 30 minutes if time allows.
Combine the remaining mint with the yoghurt, season and refrigerate until ready to serve.
Preheat greased barbecue plate on medium. Brush both sides of the patties with oil. Barbecue the patties mushrooms side up, for 10-12 minutes, carefully turn over and barbecue 8 minutes or until cooked through.
Toast the buns. Spoon some minted yoghurt onto bun bases, top with salad leaves, beetroot and lamb patties. Sandwich together with bun top and serve with more yoghurt and vegetable crisps on the side.