Preheat oven to 180°C. Grease an 8-cup lasagne dish.
Melt half the butter in saucepan on high. Sauté leek and garlic 5 minutes. Stir in flour, cook 1 minute. Remove from heat. Gradually stir in milk until smooth. Return to heat. Cook, stirring, until sauce boils and thickens. Simmer 3 minutes. Season.
Melt remaining butter in frying pan on medium. Sauté mushrooms 5 minutes. Remove from heat. Stir in soup, cream and thyme. Season.
Spread ¾ cup over base of dish. Cover with a layer of lasagne sheets. Spread half leek sauce over. Cover with lasagne sheets and half mushroom sauce. Repeat layers, ending with mushroom sauce. Sprinkle with combined cheese.
Bake 30 minutes. Set aside 5 minutes. Serve sprinkled with parsley and accompanied with salad.