Place the mushrooms in a ceramic bowl and fish on a shallow plate. Combine the olive oil, lemon juice, lemon grass, ginger, brown sugar and salt & pepper in a screw-top jar. Shake until well combined. Pour three-quarters of the dressing over the mushrooms and the rest over the fish. Stir the mushrooms and turn the fish to coat. Cover and place in the fridge for 20 minutes.
Thread the mushrooms and fish alternately onto bamboo skewers.
Preheat a greased barbecue plate or char-grill on medium-high. Cook the skewers for 5-7 minutes, turning frequently, or until fish is cooked through. Serve with tartar sauce and lemon.