Photograph: Yuki Sugiura
Preheat the oven to 200°C, fan 180°C, gas 6. Remove the mushroom stalks and chop them, then put the mushroom caps on a small baking tray. In a large pan, melt the butter and fry the chopped mushroom stalks for 2 minutes.
Add the spinach, then cover and cook, shaking the pan, until wilted. Spoon into a colander and squeeze out any excess juice. Transfer the spinach and mushroom stalks to a bowl and mix with the Boursin, a pinch of salt and a good grinding of black pepper.
Spoon the mixture into the mushroom caps and sprinkle over the breadcrumbs, mixed with the walnuts, if using. Drizzle with oil and bake in the oven for 15-20 minutes.
Kitchen secret: these make a great accompaniment to a main course, but also make an ideal vegetarian starter, allowing one or two per serving. Serve with crusty French bread.