Heat 1 tbsp olive oil in a large skillet over medium heat. Add chopped onion and sauté until softened, about 5-7 minutes. Add minced garlic and cook for another minute until fragrant. Stir in crushed tomatoes, vegetable broth, oregano, basil, red pepper flakes (if using), salt, and pepper. Bring to a simmer, then reduce heat to low, cover, and let it cook for at least 20 minutes (or longer for deeper flavor), stirring occasionally.

While the sauce simmers, heat 1 tbsp olive oil in another large skillet over medium-high heat. Add sliced mushrooms and cook, stirring occasionally, until they release their liquid and start to brown, about 7-10 minutes. Add minced garlic and cook for 1 minute more. Stir in the chopped spinach in batches, allowing it to wilt down completely. Cook until any excess liquid has evaporated. Stir in fresh parsley, salt, and pepper. Remove from heat and set aside.

In a medium mixing bowl, combine ricotta cheese, 1/2 cup Parmesan cheese, the lightly beaten egg, chopped fresh basil, salt, and pepper. Mix well until all ingredients are thoroughly combined.

Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Spread about 1 cup of marinara sauce evenly over the bottom of the prepared baking dish. Arrange a single layer of no-boil lasagna noodles over the sauce (usually 3-4 sheets, depending on the noodle size). You may need to break some to fit.

Spread half of the ricotta cheese mixture evenly over the noodles.

Spread half of the mushroom and spinach mixture over the ricotta layer.

Spoon about 1 cup of marinara sauce over the mushroom and spinach. Sprinkle with about 1/3 cup of shredded mozzarella cheese.

Place another layer of lasagna noodles. Repeat with the remaining ricotta mixture, then the remaining mushroom and spinach mixture, another cup of marinara sauce, and another 1/3 cup of mozzarella cheese.

Top with a final layer of lasagna noodles. Spread the remaining marinara sauce over the noodles. Sprinkle generously with the remaining mozzarella cheese and a little extra Parmesan cheese if desired.

Cover the baking dish loosely with aluminum foil (you might want to tent it slightly so it doesn't stick to the cheese). Bake for 30 minutes.

Remove the foil and continue to bake for another 15-20 minutes, or until the cheese is bubbly and golden brown.

Let the lasagna rest for at least 10-15 minutes before slicing and serving. This allows it to set and makes for cleaner slices.