Soak the dried porcini mushrooms in boiling water, set aside.
Wash the cremini mushrooms, cut into thick slices (about ¾ cm / ⅓ inch), mix with the oil and spices together. Arrange evenly on a baking tray lined with baking paper.
Preheat the oven to 200 °C / 400 °F / Gas Mark 6, no fan (you will need to bake the pumpkin and mushrooms separately) or 180 °C / 375 °F with fan (or using convection oven) (you can bake them at the same time).
Peel the pumpkin, using a tablespoon remove the seeds and fibers, cut into 1.5 cm / ½ inch cubes, mix with the oil and spices together, spread evenly on a second baking tray lined with baking paper.
Put the mushrooms and pumpkin in the oven (on separate baking sheets) and bake for about 25 minutes, until soft and lightly browned.
Strain the dried mushrooms, reserving the soaking water (important!), chop them finely.
While the pumpkin and mushrooms are in the oven - prepare the sauce.
Melt the butter in a medium pot, when it‘s bubbling, add the onion, cook for about 5 minutes until translucent. Add the flour, whisk it thoroughly, add the milk and mushroom soaking water gradually, after each addition (about 150 ml / 2/3 cup) mix thoroughly with a whisk, so that there are no lumps, the sauce should also thicken a bit. Remove the sauce from the heat, season with lemon juice, nutmeg, salt, and pepper to taste. Cool slightly, add lightly beaten eggs and whisk. The sauce will be rather thin, that‘s ok.
Remove the mushrooms and the pumpkin from the oven.
Finely chop the cremini mushrooms and mix with the porcini mushrooms together.
Decrease the temperature to 190 ° C / 375 °F / Gas Mark 5, no fan (if you have a convection oven reduce the temperature by about 20°).
Assemble the lasagna: Pour a little less than 1/3 of the sauce on the bottom of the baking dish (about 31x21 cm / 12 x 8 inch ). Place a layer of lasagna sheets on top, then arrange half of the pumpkin chunks, cover with another lasagna sheets layer, arrange evenly the mushrooms, pour the second 1/3 of the sauce, arrange lasagna sheets, then the rest of the pumpkin, then again lasagna sheets and pour the last 1/3 of the sauce. Sprinkle grated mozzarella cheese on top.
Cover with a piece of aluminum foil and bake for 30 mins, then uncover and bake for 10-15 min, until the cheese is golden.
Let stand for about 10-15 minutes before cutting into portions.