Brown the mince, then add the mushrooms and jar of Bolognese sauce. Simmer for 10 minutes.
Preheat the oven to Gas Mark 4/ 180?C/ fan oven 160?C. To make a white sauce, place the low fat spread, flour, skimmed milk and a bay leaf in a non stick saucepan. Cook over a medium-high heat, stirring all the time to avoid lumps, until the sauce comes to the boil. Simmer for 2 minutes then remove the bay leaf.
Spread a quarter of the white sauce over the base of an 18cm (7 inch) square baking dish. Add a third each of the Bolognese and the mozzarella and top with two sheets of lasagne.
Repeat to make two more sets of layers of Bolognese, mozzarella and lasagne. Pour the rest of the white sauce evenly over the lasagne and scatter the parmesan cheese on top.
Bake in the centre of the oven for 40 minutes or until the pasta is tender and the top of the lasagne is golden brown and crisp at the edges.