Preheat the oven to 180C/350F/Gas 4.
Place the mushrooms open-side up on a baking sheet and sprinkle over the chopped garlic and cheese. Drizzle with oil.
Transfer to the oven and bake for 5-6 minutes, or until softened and golden-brown.
Meanwhile, heat the butter in a frying pan, add the spinach and stir until wilted. Season, to taste, with salt and freshly ground black pepper.
Remove the mushrooms from the oven and serve with the wilted spinach piled on top.