Pre-heat oven to 400F, Gas Mark 6, 200C
Place the carrots, celery, leeks and broad beans in a large pan and cover with boiling water
Cover, bring to the boil and simmer for 5-7 minutes until the vegetables are tender
Drain off the water, reserving half a pint (285 ml) for the sauce)
Melt the butter in a pan then stir in flour and cook gently for 1 minute while stirring
Gradually add the milk and vegetable stock, stirring continuously until the sauce comes to the boil and thickens
Stir in the cheese, cooked vegetables, mushrooms and sweetcorn
Season well and set aside
Cook the lasagna according to packet instructions then drain and refresh in cold water to prevent over-cooking
Layer lasagna in a shallow ovenproof dish alternating with the vegetable sauce, finishing with sauce
Sprinkle with grated cheese as a topping and bake in the pre-heated oven for 30 minutes or until the top is golden