Combine the oil, vinegar, mustard, sugar and ginger into a screw-top jar, season with salt and pepper. Shake will to combine.
Place the mushrooms into a large bowl. Pour over three-quarters of the dressing and toss to combine. Cover and set aside to marinate for 1 hour.
Arrange lettuce over base of a serving platter. Top with prawns, avocado and marinated mushrooms. Spoon over the remaining dressing, scatter with mint leaves, season with salt and pepper. Serve with crusty bread and lemon.