Preheat the oven to 375°F (190°C).
Bring a large stock pot with 4 quarts water and 1 tablespoon salt to a boil. Add the lasagna noodles and cook until just al dente, about 10 minutes. Drain in a colander and rinse with cold water to prevent sticking.
Heat 2 tablespoons olive oil in a large saucepan or Dutch oven over medium-high heat until shimmering. Add the sliced mushrooms, chopped onion, and 1/2 teaspoon salt and cook, stirring occasionally, until the mushrooms have released most of their liquid, about 10 minutes.
Add the minced garlic to the pan and cook until fragrant, about 30 seconds.
Stir in the squeezed spinach, crushed tomatoes, tomato sauce, granulated sugar, dried basil, Italian seasoning, and fennel seeds. Bring to a simmer and cook uncovered for 15 minutes, stirring occasionally. Taste and adjust salt and pepper (about 1½ teaspoons salt and ½ teaspoon black pepper total is suggested).
While the sauce simmers, in a large bowl combine the ricotta cheese, egg, dried parsley, and 1/2 teaspoon salt; mix until smooth and chill until ready to layer.
Spoon 2 cups of the sauce into the bottom of a 9x13-inch baking dish.
Arrange a single layer of cooked noodles over the sauce. Spread a heaping 2/3 cup of the ricotta mixture over the noodles, sprinkle about 1½ cups (roughly 5 ounces) of shredded mozzarella, and about 1/3 cup grated Parmesan.
Spoon 2 cups of sauce over the cheese, then repeat the noodle, ricotta (heaping 2/3 cup), mozzarella (1½ cups), and Parmesan (1/3 cup) layers two more times for a total of three layers.
Spray a large piece of foil with nonstick spray and cover the baking dish. Bake covered for 15 minutes, then remove the foil and bake uncovered for an additional 25 minutes, or until hot and bubbly.
Let the lasagna cool at least 10 minutes before slicing and serving.
