Melt the butter in the saucepan, sauté the leek and onion and cook for about 4 minutes. Add the garlic and cabbage and cook for another 10 minutes until the cabbage has collapsed. Pour in the broth and simmer for 10 minutes. Season to taste with salt and pepper.
Place the bread on the bottom of a deep ovenproof dish and sprinkle with half of the grated cheese. Pour the soup over the bread and sprinkle with the remaining cheese. Bake in the preheated oven at 180 degrees for about 1 hour.