Add pancetta to a large saute pan set over medium heat.
Chop mushrooms roughly, slicing any meatier mushrooms. Add to pan with pancetta and stir to combine. Cover and allow to saute, stirring sporadically to distribute the liquids releasing from the mushrooms. Add a splash water to assist with this if your mixture is still rather dry after 3 minutes.
Meanwhile, set a saucepan filled with the milk over medium low heat. Add 4 sage leaves to the mixture, ensuring they're immersed. You're infusing the milk, so be sure to stir the mixture every so often.
Once mushrooms have softened and released their liquid, remove from the pan and set aside.
Add 1 stick of butter to the same saute pan, allowing to completely melt. Whisk the butter as you add in 1/2 cup of flour. The mixture will seize up a little, but keep whisking! It should thin out after 2 minutes.
Add warm milk, 1 cup at a time and whisking constantly, until you have a thin sauce. Add smoked mozzarella, again whisking until it fully melts, followed by the parmesan. Add 1 teaspoon of salt a some freshly grated nutmeg (start with a tiny pinch). Taste the sauce and adjust to your liking.
When ready to assemble, preheat the oven to 400ยบ. In a 13 inch baking dish, add enough sauce to completely cover the bottom of the pan. Add lasagna to cover the bottom, followed by half of the mushroom mixture. Top with about 1/2 cup of the sauce - you still have one more layer to go! Repeat with lasagne, mushroom mixture, and sauce until all components are gone.
Top with smoked and regular mozzarella cheese. Bake for 30-40 minutes until bubbling and lightly golden (you can opt to broil the top at the very end if you'd like more of a golden brown top).
In the last 20 minutes of baking, toss the remaining mushrooms with 2 tablespoons of olive oil and season with salt and pepper. Roast for 20 minutes until softened and lightly golden. Removefrom oven with the lasagna and allow to slightly cool.
Add remaining olive oil to a small saute pan over medium heat. Drop your sage leaves in to lightly fry: the oil will be ready when it starts to sizzle when you drop a sage leaf in. Fry for about 1 minute per side, until lightly crispy but still green.
Top lasagna with roasted mushrooms and fried sage. Serve warm!