Heat two large non-stick sauté pans to a medium-high heat. Add one tablespoon of butter into each. Add mushrooms. Toss in butter. Let mushrooms brown, stirring occasionally. This should take about 7-10 minutes. Once the mushrooms are brown and slightly reduced in size, combine them all into one pan. Add the sage, sherry and salt. Simmer until sherry has reduced. Season with salt and pepper if necessary. Set aside.
For the ricotta:
Line a sieve with cheesecloth or paper towels. Pour milk and salt into a large pot. Bring the milk to a boil, and then reduce to a simmer. Add in lemon juice and using a wooden spoon, stir until the mixture separates and you are left with curds. Once the mixture is completely separated strain into cheese-cloth lined sieve. Let strain until ready to use.
For the béchamel:
In a medium sized sauce pan, heat butter over a medium heat. Whisk in flour, cook one minute. Slowly whisk in milk, making sure to get all lumps out. Add salt. Bring to a boil and reduce to a simmer until thickened, about five minutes. Season with salt and pepper if necessary.
For assembly:
Spray a rectangle 13x8 baking dish, with non-stick cooking spray. Spread an even layer of béchamel on the bottom of the pan. Top with a layer of the spinach pasta. Spread half of the mushrooms over the pasta, followed by half of the ricotta. Sprinkle with ⅓ of the fontina. Top with another layer of béchamel. Repeat once. For the last layer, place another layer of lasagna sheets on top of béchamel, pour remaining béchamel on top, following by the remaining fontina. Lay slices of mozzarella of top.
Cover with foil and bake for 40-45 minutes or until the lasagna sheets are cooked and the cheese is melted. Remove the foil, turn the oven to broil and place back in the oven until the top is golden brown and bubbly.
Recipe by Cooking for Keeps at https://www.cookingforkeeps.com/triple-mushroom-lasagna-ricotta-sage-fontina/