Place the mushrooms, gill side up, in a large roasting tin, drizzle with 1 tablespoon of the oil and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 6–8 minutes.
Meanwhile, heat the remaining oil in a large frying pan, add the spring onions and sauté for 3–4 minutes. Gradually add the spinach, stirring until wilted. Add the rice and half the cheese and season to taste, then mix well.
Remove the mushrooms from the oven and divide the spinach mixture between the caps. Sprinkle with the remaining cheese and cook for a further 8–10 minutes.