Preheat the oven to 190°C, fan oven 170°C, Gas Mark 5.
Put the mushrooms, gills facing up, into a roasting tin.
Take 8 slices from the aubergine (crossways, not lengthways), then chop any remaining aubergine finely.
Heat half the olive oil in a large frying pan and briefly fry the aubergine slices for 1-2 minutes on each side. Remove from the pan.
Add the rest of the olive oil to the pan and cook the onion, pepper and chopped aubergine for 5-6 minutes. Add the tomatoes and tomato puree. Cook gently for another 1-2 minutes, then season with salt and pepper.
Spoon half the tomato mixture into the mushrooms and top with a slice of aubergine. Spoon the rest of the tomato mixture on top, then sprinkle with the vegan Mozzarella. Transfer to the oven and bake for 20-25 minutes.