In a large cast-iron pan, sauté onions with 1 teaspoon of sugar, ½ teaspoon salt, and 1 teaspoon each of black pepper, red pepper, and dried oregano. Cook until golden.
Transfer the onions to a bowl and in the same cast iron, sauté the mushrooms with a drizzle of olive oil. Season again with salt and pepper to taste, red pepper, and oregano.
Once the mushrooms are golden, add the sautéed onions and garlic and a tablespoon of olive oil. Cook for another 1-2 minutes. Transfer the mushroom and onion mixture to a large bowl.
Again, using the same cast-iron pan, prepare your sauce. Add the tomato sauce with a drizzle of olive oil, and season with sugar, salt, and pepper to taste, red pepper flakes, dried oregano, and shaved Parmesan.
Let the sauce simmer for about 15 minutes. Add the mushroom mixture to the sauce, and stir to combine. Set aside.
Prepare lasagna noodles as directed. Set aside.
Preheat oven to 350 degrees F.
In a 9 x 13 lasagna dish, start with 2 teaspoons of olive oil and about 2 tablespoons of sauce at the bottom of your dish. Add a layer of lasagna noodles, a layer of sauce, ricotta cheese filling, grated mozzarella, shredded Parmesan, and then a layer of the fresh spinach.
Repeat this process until the pan is filled. Top with one more layer of sauce, the three cheeses, and then a drizzle of olive oil.
Bake for about 30 - 35 minutes or until golden and bubbly. Let cool slightly before serving and enjoy!