Preheat oven to 180 degrees C. Bring a saucepan of lightly salted water to the boil. Add lasagne and cook for 8 to 10 minutes or until al dente; drain.
In a small saucepan, saute onion and mushrooms in chicken stock until softening. Remove from heat. Stir in soup, milk and basil. Mix well and set aside.
In a lightly greased 20x30cm baking dish, arrange 3 cooked sheets lasagne. Layer half the chicken, half the ricotta cheese, half the Cheddar cheese and half the mushroom mixture. Add the other 3 sheets lasagne and repeat layers. Bake uncovered in the preheated oven approximately 50 minutes.