Melt the butter in a saucepan then add the cornflour.
Slowly, bit by bit, add in little bits of the milk. Stirring as you add.
Once the milk has nearly all been used, add the cheese in and final bit of milk. Stir constantly until sauce has thickened.
Pre-heat oven to 190 degrees. Chop onion, garlic, courgette, mushrooms and ham.
Layer lasagne sheets at the bottom of a large lasagne/baking dish. Ensuring all is covered.
Make the white cheese sauce using the instructions above. Mix together the onion, garlic, courgette, mushrooms and ham with the sauce.
Place and spread the mixture on top of first layer of lasagne sheets. Arrange lasagne sheets on top as before.
You should be able to make at least 1/2 more layers by repeating these steps.
Top with 50g of grated cheddar cheese.