Finely chop the Portabellinis and sweat off with about a tablespoon each of olive oil and butter, with salt added early to help extract the moisture and pepper at the end.
Cook the ostrich mince separately in the same way (As the moisture evaporates, so the mushroom juice is concentrated and, combined with the oil and butter, it gives a sheen to the mushrooms.
Hand blend the mix of ostrich mince and chopped mushrooms and shape into a burger sized patty.
To serve
Toast the buns on the inside and stack as follows: baby lettuce leaves, a layer of pink sauce, 3 slices pickled gherkin, 2 slices melted Emmental cheese, one tablespoon of onion marmalade and then the mushroom ostrich patty on top.
Garnished with a rosemary spear, battered onion ring, grilled Portabellini and a cherry tomato.