Pour the boiling water into a wide pan, then stir in the bouillon powder. Add the mushrooms and poach for 2–3 minutes, then remove with a slotted spoon and drain on kitchen paper.
Heat a little of the oil in a pan, add the onion and cook until soft. Remove from the heat and leave to cool.
Mix together the onion, tofu, pine nuts, cayenne pepper, basil and the remaining oil. Season well with salt and pepper.
Sprinkle some Parmesan over each mushroom, then stuff the mushrooms with the onion mixture. Arrange in a flameproof dish and cook about 15 cm (6 inches) below a preheated medium grill for 10 minutes or until heated through and the cheese has melted.
To serve, scatter the spinach leaves on a plate and arrange the hot mushrooms on top.