Heat the butter in a saucepan. Add the garlic and sauté until it just starts to colour. Add the mushrooms and toss briefly. Add lemon juice and season with salt and pepper to taste.
Toast the bread and brush the top lightly with some butter. Layer with salad leaves and cucumber; spoon the sautéed mushrooms over. Sprinkle with a little extra coarsely ground black pepper and garnish with shredded carrot. Serve at once.