1. Preheat the oven to 200C/gas mark 6. Heat one tablespoon of olive oil in a large pan, add half the mushrooms and fry until golden, about five minutes.
2. Transfer them to kitchen paper, add another tablespoon of oil to the pan and fry the remaining mushrooms, also transferring them to kitchen paper after five minutes.
3. Add the final tablespoon of olive oil to the pan and cook the onion and garlic for about four minutes, until soft. Add the spinach to the pan, cover with a lid and allow it to wilt for one to two minutes. Return the mushrooms to the pan, mix together and season.
4. Use a rectangular dish about 24 x 18cm and spread a layer of white sauce on the base. Place three lasagne sheets on the sauce followed by a third of the mushroom mixture, another layer of white sauce and some crumbled cheese. Add another three sheets of lasagne and continue layering, finishing with the final three sheets of lasagne, a good covering of white sauce and a sprinkling of crumbled cheese.
5. Bake for 35 to 40 minutes until it is cooked through and the topping is bubbling and golden. Serve with a side salad or garlic bread.