To make the dressing. whisk miso paste and lemon juice together and set aside.
Puree the avocado, mustard and mayonnaise in a blender. Set aside.
Preheat a greased barbeque grill on high heat. Toss mushrooms in the oil and season with salt and pepper. Arrange the mushrooms stem- side down on the barbeque for 3 minutes. Turn the mushrooms and spoon over the dressing. BBQ for 3 - 4 minutes until the sauce is hot and the mushrooms are tender.
Place the salad greens on serving plates, top with avocado puree, mushrooms and sprinkle with the beetroot then serve.