Peel and dice the onions; wash the carrots and dice. Wash and slice the mushrooms; trim the courgettes and slice thinly. Grate the cheddar for the sauce.
Saute the onions, carrots, mushrooms and courgettes in the remaining olive oil for a few minutes and then add the herbs, tomato base sauce and lentils. Cook for 10-15 minutes - the lentils should be just cooked.
Make the cheese sauce by melting the margarine then adding the flour. Cook until the texture and colour are sandy then add the milk, stirring continuously until thickened and smooth. Add the cheese and mustard powder and cook for a further 2-3 minutes.
Pour half the vegetable mixture into the tin, top with a layer of lasagne. Repeat the process and then pour over the cheese sauce.
Bake in the preheated oven until golden brown and the lasagne is soft - about 20 minutes.