If you can't find no-boil lasagna noodles, you can use regular lasagna noodles, no problem. Boil them until al dente (firm to the tooth), strain, and shock with cool water, then line them up on a lightly oiled baking sheet before assembling the lasagna. That's how they do it in restaurants!
Preheat the oven to 375°F. Lightly spray a 9 x 13- inch baking dish with olive oil cooking spray and set aside.
In a large sauté pan, warm the olive oil over medium- low heat. Add the onion and cook until translucent, 6 to 8 minutes. Add the mushrooms, spinach, and cook for an additional 5 minutes, stirring occasionally. Remove from heat, and let cool slightly.
Place the ricotta cheese in a large bowl, and, using a spatula, gently fold in the cooled mushroom-spinach mixture until completely incorporated.
Assemble the lasagna
Using the prepared baking dish, assemble the lasagna in six layers as follows: 1 cup Homemade Marinara, spread evenly; 3 lasagna noodle sheets (arranged in a single layer); ¾ cup of the spinach-mushroom- and-ricotta mixture, spread evenly. Repeat the layering a second time, then top with the remaining spinach-mushroom-and-ricotta mixture, and finish with the grated Pecorino or Romano on top. Cover with aluminum foil, and bake in the preheated oven, on the middle rack, for about 25 minutes.
After 20 minutes have passed, remove the foil from the baking dish and move the baking dish to the top rack. To finish, bake, uncovered, for another 5 to 10 minutes, depending on how you love your lasagna finished.
Remove the lasagna from the oven and cool for 5 to 10 minutes before cutting and serving.