Bring abundant water to the boil in the biggest pot you own.
In the meantime, if your mushrooms aren't already sliced, slice them now (not too thin).
Cook lasagna sheets in boiling water until al dente, moving them around often with a fork or spoon to avoid from sticking together.
Once lasagna sheets are cooked, don't drain them in a colander but rather fish them out one by one and lay them out on non stick baking sheets or plates or even just on parchment paper laid out on your kitchen counter to avoid from having them stick together.
Preheat oven to 350F (180C)
Heat a large pan over medium high heat.
In a another medium saucepan melt butter over medium low heat. Once melted add flour and whisk until all lumps are dissolved. Add 750 ml milk, 2 tsp salt, 1/8 tsp nutmeg and pepper to taste and stir, and stir some more and some more and some more. Do not stop stirring until the sauce has thickened to about yogurt consistency, then remove from heat and keep stirring until it cools down a little.
Now the large pan should be smoking hot, add a splash of olive oil and then sliced mushrooms, a good amount of salt and pepper and move around quickly to avoid from burning and to ensure all the water evaporates quickly so the mushrooms don't become soggy.
Now it's time to assemble the lasagna. First spread a very thin layer of bechamel sauce on the bottom and then lay out lasagna sheets to cover the whole bottom, cut the sheets accordingly to fit in. The first layer of bechamel is not for taste but to avoid the lasagna sheets from sticking to the bottom so it really only has to be very thin.
Now spread a thicker layer of bechamel sauce on the lasagna sheets covering edges and corners really well, too.
Lay out artichoke leaves in a single layer, then sprinkle a little bit of cheese on top.
Now start your second row by laying out lasagna sheets, spreading bechamel sauce on them, placing sliced and sautéd mushrooms in one layer on top, sprinkling with cheese and top with lasagna sheets.
Another row with artichokes again and one you finish with lasagna sheets again we will just finish off the lasagna with one last layer of bechamel sauce and sprinkle with Parmesan cheesehere again the oder: bechamel-pasta-bechamel-artichoke-cheese-pasta-bechamel-mushroom-cheese-pasta-bechamel-artichoke-cheese-pasta-bechamel-Parmesan
Put in the oven for 30 minutes and finish off with 3-4 minutes under the broiler to get a dark and crispy top.