Preheat oven to 180C/350°F Grease an 8 cup lasagne dish.
Melt half the butter in a saucepan on high. Saute leek and garlic for 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, saute mushrooms 5 mins, remove from heat. Stir in soup, cream and thyme, then season.
Spread 1/2 cup of mushroom mix over base of lasagne dish and cover with a layer of lasagne sheets. Spread 1/2 of leek sauce over. Cover with lasagne sheets and 1/2 of remaining mushroom sauce. Cover with lasagne sheets, then other half of leek sauce. Top with lasagne sheets and remaining mushroom mix and cheese.