Preheat oven to 375 degrees.

Bring a large pot of salted water to a boil. Cook lasagna sheets according to package directions. Drain and set aside.

Meanwhile, cook butternut squash in a large pot on medium heat with 1 Tbsp. olive oil, salt, and pepper. After a few minutes, when squash starts to cook, add 1/2 cup water and let squash cook until soft. Once cooked, add to a food processor and puree.

Cook garlic and mushrooms in 1 Tbsp olive oil until mushrooms start to cook through. Add spinach and toss until spinach begins to wilt.

Prepare the sauce: In a small saucepan, melt butter on low heat. Whisk in flour. Slowly add milk and whisk to remove any lumps. Allow sauce to thicken over low heat, stirring often (about 5 minutes) - be careful not to boil. Remove from heat and stir in fontina cheese and half of the parmesan. Whisk until smoothy and creamy.

Layer the lasagna: On the bottom of a large baking dish, pour half the cheese sauce, and line bottom of the pan with one layer of lasagna sheets. Top with 1/3 of the butternut squash puree, 1/3 of the vegetable mixture, and 1/3 of the mozzarella cheese. Top with another layer of lasagna sheets and repeat 2 more times, finishing with a final layer of lasagna sheets. Pour remaining cheese sauce over the top layer, and finish with remaining parmesan.

Cover tightly with foil and bake for 45 minutes. Let stand for 15 minutes before serving.