If necessary use a small knife to trim the mushrooms to form a larger cavity for the stuffing.
Combine avocado, 1 tbs lemon juice, chilli and green onions. Season with salt and pepper and stir gently to combine.
Combine the yoghurt, tomato sauce, Worcestershire sauce and 2 tsp lemon juice together.
Spoon the avocado filling into mushroom cavity. Top with a little lettuce then a prawn. Arrange on a platter drizzle with a little sauce and serve.