Cut the smoked sausage into small cubes. Heat the oil in a pan and gently fry for 4-5 minutes.
Add the tomato and herb pasta sauce into the pan and stir to heat through.
Put a third of the smoked sausage mixture in an oven proof dish, then a layer of lasagne sheets, then a third of the white sauce. Add another layer of sausage, lasagne and white sauce. Repeat these layers once more, making sure that there is enough white sauce to cover the top, then top with the grated cheese.
Bake in a pre-heated oven until bubbly, about 30 minutes. Leave to rest for approximately 10 minutes. This stops the lasagne from falling apart when serving.