For the mushroom lasagna, clean and slice the mushrooms. Save some nice slices for decoration, fry the rest in olive oil until brown.
Chop the onions, add to the mushrooms in the pan and sauté until translucent. Add marjoram and season with salt.
Sauté flour in olive oil, add soy milk and simmer for a few minutes while stirring. Mix in porcini powder and simmer for 1-2 minutes, adding more soy milk if necessary.
Remove the sauce from the heat, stir in the cheese and season with salt. Preheat oven to 180 °C top and bottom heat. Brush the casserole dish with a little olive oil and place the first layer of lasagne sheets in it.
Spread some fried mushrooms on the lasagne sheets and cover with bechamel sauce. Continue in the same way until the mushrooms are used up and there is still some bechamel sauce left.
Cover the last layer of lasagna sheets with the remaining bechamel sauce, sprinkle the grated cheese and place the beautiful mushroom slices decoratively on top.
Bake the lasagna for about 45 minutes, until the surface is nicely browned. Let it rest for another 10 minutes before serving.
If you do not have porcini powder, you can also omit it. The mushroom lasagna then tastes less intensely of mushrooms. In the porcini season, nman can prepare the lasagna with roasted porcini mushrooms, that tastes particularly fine.