Preheat the oven to 180 degrees Celsius.
Place the porcini mushrooms in a saucepan with water and bring to the boil. Reduce the heat to simmer for 10 minutes until the mushrooms have softened.
Remove them from the liquid using a slotted sieve, retaining the liquid to use in the sauce.
Chop the porcini finely. Sauté the brown and button mushrooms in oil with the shallots and porcini for 5 minutes. Mix the arrowroot in some of the porcini water to make a smooth paste. Add it to the mushrooms with the remaining the porcini water and chopped chervil and simmer for a further 5 minutes.
In a food processor, blend the ricotta, skimmed milk, nutmeg and sea salt until smooth.
Lightly oil a baking dish and line the base with a layer of pasta, followed with a layer of mushroom, pesto and another layer of pasta. Continue until all the ingredients are used up. Top with the ricotta sauce. Sprinkle parmesan over the top. Cover with foil and bake in the oven for 25 minutes. Remove the foil; increase the heat to 200C and cook for a further 5 minutes until the parmesan is golden.
Serve with a green salad.
Note: Instant spelt lasagna sheets are available from health food stores. Apart from saving on one step of the lasagna making process it is more suited for people who have MILD gluten intolerance. If you can't find it and gluten is not a problem, substitute it with wholemeal pasta and cook it for 6 - 8 minutes before use.