Preheat the oven to 350°F (177°C). Grease a 9×13-inch baking dish with a small amount of oil.

Heat the olive oil in a large skillet over medium heat. Add onions and mushrooms, cooking until onions are translucent, about 10 minutes. Add garlic, spinach leaves, and ¼ teaspoon of salt, cooking until spinach is bright green and wilted.

In a medium bowl, combine ricotta cheese, eggs, ½ teaspoon of salt, and ¼ teaspoon of pepper until well blended.

Spread 1 cup of pasta sauce over the bottom of the baking dish. Arrange three noodles to cover the bottom. Spread half of the ricotta mixture over the noodles. Sprinkle with one-third of the parmesan and one-third of the mozzarella. Add half of the spinach mixture, then top with about ½ cup of marinara sauce. Add a second layer of noodles and repeat with remaining ingredients. Finish with a third layer of noodles and top with the remaining sauce, parmesan, and mozzarella.

Loosely cover the lasagna with aluminum foil and bake for 30 minutes. Uncover and bake an additional 10 to 15 minutes until cheese is bubbly and browned. For a golden-brown top, broil for 1 to 2 minutes. Let sit for 5 to 10 minutes to set before serving.