Preheat the oven to 230c / 210c fan / 450f / gas mark
Peel the mushrooms, season with black pepper and set aside.
Wilt the spinach. Rinse it in a colander, then pour a kettleful of boiling water over it to wilt it. Finish with cold water to cool it, then squeeze as much water out as possible, then chop and spread over the mushrooms.
Sprinkle some nuts over each mushroom, then top with slices of avocado and some goats cheese. Season with pepper and bake for 15 minutes until the mushroom is cooked.