Wash the mushrooms and let them soak in enough cool water. Then bring to the boil a pan with 3 litres of very salted water.Pass the lasagne with spinach in boiling water with the sesame oil and cook them al dente.Drain them and plunge them in cold water, drain them again and place them on a tea towel.Clean and rinse the celery, the carrot, the onion, dry them and chop them very finely. Place the chopped vegetables in a very large saucepan and slather with oil.Fry lightly for a few minutes and add the peeled sausage, all the minced meat, and after having mixed the lot well with a fork, add the mushrooms with their water.Finely melt the tomato concentrate the vegetable broth and mix it with a smattering of thyme and the whole leaves of basil.Mix well all these ingredients and simmer over a very gentle heat for 45 minutes, stirring from time to time.Next, prepare the béchamel in a small pan melt 50 g of butter.First add the flour, constantly stirring to prevent lumps, then the cold milk; then bring the sauce to the boil, adding another dash of milk in case it thickens too much; sprinkle a pinch of nutmeg, season with salt and pepper and remove the sauce from the heat.Lightly butter grease with a deep oven-proof dish. Arrange a layer of spinach lasagne, spread a spoonful or two of the prepared sauce and grated parmesan, place another layer of lasagne, which you will cover with béchamel and a sprinkle of grated parmesan.Repeat this process, alternating layers of lasagne with the sauce and the béchamel until the ingredients are finished.Leave enough bèchamel to cover last layer of lasagne; for a better presentation, scatter a few dollops of sauce over the lasagne.Put a few flakes of butter over the bèchamel, sprinkle it with grated parmesan and breadcrumbs.Put the dish in a preheated oven at 200 °C for about 20 minutes until the top is au gratin. Serve warm but not hot in the same oven-proof dish.