Preheat the oven to 180°C. Spray a large frying pan with low-calorie cooking spray and place over a medium heat. Add the onion, leek and garlic and cook for 5 minutes. Dissolve the stock cube in the boiling water.
Add the mushrooms and cook for a further 5 minutes until softened. Add the chicken and cook for 5 minutes until sealed on all sides. Stir in the mustard powder, Dijon mustard, chicken stock and thyme and simmer gently, uncovered, for 10 minutes until the chicken is cooked and no longer pink inside.
Remove from the heat and stir in the cream cheese until completely blended, to form a filling with a creamy sauce. Season with salt and pepper and set aside.
Pour the milk into a small saucepan, add the reduced-fat spread and heat gently until steaming hot and the reduced-fat spread has melted. Take care not to let the milk boil over.
Mix the cornflour with 3 tbsp of cold water in a small bowl or cup until smooth, then pour the cornflour mixture into the hot milk, stirring constantly with a balloon whisk or wooden spoon. Simmer for 3 - 5 minutes, stirring, until the sauce is smooth and thickened. Stir in the mustard powder and grated cheese until melted. Season with salt and black pepper, if needed, and remove from the heat.
Place half of the chicken, mushroom and leek filling in an 18 x 27cm ovenproof dish and spread out evenly.
Place 3 lasagne sheets over the filling, trimming to fit if needed. Pour half of the cheese sauce over the lasagne sheets and spread out evenly.
Place the remaining filling on top and spread out evenly. Place the remaining 3 lasagne sheets over the filling, trimming to fit if needed. Pour the remaining cheese sauce over the lasagne sheets and spread out evenly.
Sprinkle the grated cheese over the top of the lasagne and place on a baking tray. Bake in the preheated oven for 40-45 minutes until the lasagne is tender when tested with a small sharp knife and the top is golden brown.
Serve with a crisp mixed salad or other accompaniment of your choice.