Clean the mushrooms by wiping with a damp cloth and slice up. Heat the oil till very hot and fry the mushrooms in batches till golden. Set aside.
Heat a little more oil and gently fry the onion, garlic and thyme till soft.
Wilt down the spinach by adding a handful at a time to the hot pan and stirring it in. The heat will wilt down the leaves, just be patient.
Remove the thyme twigs. Mix with the cooked mushrooms, season with salt, pepper and nutmeg.
To make the white sauce, melt the butter in a pot. Stir in the flour and cook out for a minute. Gradually add the milk and whisk to remove any lumps. Boil for 2 minutes. Season with salt, pepper and nutmeg.
Grease a lasagne dish. Pour in enough white sauce to cover the bottom of the dish. Place down a pasta layer. Spoon over half the spinach and mushrooms. Repeat, finishing with a layer of pasta.
Cover the top layer very well with white sauce. Sprinkle over the cheese.
Bake at 200°C for 30 to 40 minutes until golden and bubbling.