Preheat the oven to Gas Mark 6, 200ºC (400ºF). Place the mushrooms in a shallow roasting tin and sprinkle with the thyme, garlic, olive oil and lemon juice. Roast for 10 minutes.
Meanwhile, heat the butter in a large pan, add the spinach and stir-fry it over a high heat until it wilts down into the bottom of the pan. Tip into a colander and press out the excess liquid. Season to taste with salt and pepper.
Remove the mushrooms from the oven and top with the spinach and sliced cheese. Return to the oven for 3-4 minutes, until the cheese has just melted.
Meanwhile, toast the bread until golden. Rub each slice with the whole garlic clove and drizzle with a little olive oil.
Place each slice of toast into the centre of a warmed plate and place 2 of the mushrooms on top. Spoon over the juices from the roasting tin, and serve.