Preheat a roasting pan and oven to 240°C or 220°C fan forced. Spray mushrooms all over with oil. Remove the hot pan from oven and quickly add mushrooms. Squeeze over the lemon juice, season to taste and turn to coat.
Roast mushrooms 10-15 minutes or until tender but still hold their shape. Remove mushrooms to a plate and cool to room temperature.
Combine crab meat and mayonnaise. Spoon a little avocado into each mushroom. Top with a spoonful of crab then scatter with micro herbs.