Roast garlic by cutting the top of the bulb off, exposing the tops of the garlic cloves (reserve trimmed garlic tips -- about 1 teaspoon, minced);
place bulb in a small oven safe dish, drizzle enough oil over garlic bulb to coat the top and surface; roast in oven at 375° for 30-40 minutes until garlic is fragrant, lightly browned, and soft. Remove from oven and allow to cool until easy to handle.
To remove the roasted garlic from the bulb, squeeze the bulb at the bottom releasing the cloves out the cut end -- cloves should pop right out. Coarsely chop and mix with the mushrooms, once cooked. Reserve any remaining oil from the roasted garlic to grease the baking pan.
Trim ends from squash, then slice a sliver off one side to create a flat side. Stand squash up on a cutting surface like an ear of corn and carefully slice the squash lengthwise along the flat side into 1/8" slices, running a knife just inside the original trimmed edge to create flat, wide "noodles".
Lay out squash on a paper towel lined pan, sprinkle with salt, and let set 30 minutes to sweat. Once garlic is roasted, increase oven temperature to 400°F. Brown sausage in a large skillet over medium heat.
Once browned, mix the two types of sausage together and set aside. Using the fat from the sausage (add oil if needed), cook the mushrooms and garlic over medium heat until the mushrooms are wilted and release their moisture.
Transfer mushrooms to a paper towel lined bowl and toss with roasted garlic. Set aside until ready to use.
Rinse zucchini and pat dry with paper towels. Arrange zucchini on a rack set over a rimmed baking sheet and bake for 15 minutes. Remove from oven and reduce oven temperature to 350°F.
Mix the ricotta cheese, egg, parsley, and nutmeg until well blended. Stir in 3/4 cup of the Parmesan and 2 cups of the mozzarella.
Lightly grease a 9x13 pan and layer with 2/3 cup sauce, zucchini, half the sausage, half the mushroom and roasted garlic, 1/2 the basil, 1 cup mozzarella, and all the ricotta mixture.
Continue to layer with 2/3 cup sauce, remaining zucchini, sausage, mushrooms and roasted garlic. Top with 1 1/2 cup sauce, 2 cups mozzarella, and remaining basil
Bake for 35-40 minutes, or until cheese is melted and golden brown on top. Remove from oven and let stand for 10 minutes before serving.
This lasagna will release more liquid than lasagna made with pasta. Serve with crusty French bread to soak up every drop.