Preheat oven to 375 degrees
Peel and slice the sweet potatoes about 1/4 inch thick and place them on a parchment lined baking sheet. Lightly brush the slices with olive oil on both sides and season with kosher salt and pepper.
Bake for 20 minutes, flipping them over after 10 minutes.
Slice the cleaned portobellos toss them with about 2 tablespoons of olive oil and lay them in a single layer on a baking sheet. Place them on the bottom rack of the oven for 15-20 minutes. If you find they have released liquid don't worry add it to the lasagna. Since I'm already roasting the sweet potato I do the mushrooms this way. Fell free to sauté them in a pan if you find that easier.
While the veggies are roasting, prepare the béchamel. In a medium sauce pan melt the butter on medium heat. Once butter has melted and starts to bubble add flour and whisk to form a paste (roux). Add half of the milk and whisk until mixture thickens. It should be pretty thick at this point. Add the remaining milk continuing to whisk until it returns to boil and thickens a bit. Remove the pot from the heat and add salt , cheese, nutmeg and mix well.
Reduce oven temperature to 350
Grease a 9 x 13 baking dish with cooking spray. Spread 1/2 cup of the tomato sauce evenly on the bottom of the baking pan. Layer your lasagna sheets(3-4) to cover the bottom. Add another 1/2 cup of sauce, 1/2 cup of bechamel, 1/4 cup Parmesan cheese. This will be your formula for each layer of your lasagna. *tip: If you find you need to stretch your tomato sauce simply add a bit of water to it.
On layer 2 cover sauce with half of the spinach
Layer 3 add the sliced sweet potato
Layer 4 add the sliced mushrooms
Layer 5 add remaining spinach
Layer 6 tomato sauce / bechamel and parm
Top with more noodles spread the rest of the bechamel, Parmesan and top with the 2 cups of shredded or sliced mozzarella.
I like to season the top with salt, pepper and oregano but it's optional.
Bake uncovered for 45 minutes. Let the lasagna rest for at least 20 minutes before serving. Enjoy!
