Bring a large pot of water to a boil and line a sheet pan with parchment paper. Add 2 teaspoons of salt, then add lasagna sheets and cook until al dente. Drain lasagna sheets, then set overlapping on the prepared sheet pan so they don’t stick together.

Add olive oil to a large pot and set over medium heat. Add onion, green pepper and mushrooms and cook for 12 to 15 minutes, or until the vegetables have softened and the mushrooms have released all of their moisture. Add the spinach, garlic, and remaining 1 teaspoon salt, and cook for 2 to 3 minutes more. Set vegetable mixture aside.

In a medium bowl combine ricotta, egg, Italian seasoning, black pepper, and a pinch of salt.

Preheat the oven to 375˚F. Add a very thin layer of marinara sauce into the bottom of a 9 by 13-inch baking dish. Add a layer of noodles, followed by about 1 ½ cups of the vegetable mixture, doing your best to smooth the veggies into a single layer. Dollop about ¾ cup of ricotta mixture on top of the veggies, smoothing it into a somewhat even layer. Add a thin layer of marinara on top of the ricotta mixture, then top with about ½ cup of mozzarella, and finally another layer of noodles. Repeat this process two more times so you have a total of three layers. Add remaining marinara sauce on top of the final layer of noodles, followed by remaining mozzarella and all of the grated parmesan cheese.

Cover with foil and bake in the preheated oven for 30 minutes. Remove the foil, and bake for an additional 20 minutes or until the cheese is golden and bubbly. Let rest for 15 minutes before cutting.