Heat the oven to 375°F and grease a 9 x 13-inch baking dish. If using regular noodles, boil them in salted water until just al dente, drain, and lay them flat so they do not stick. Prepare oven-ready noodles according to the package directions.

Heat the olive oil in a large skillet over medium heat. Cook the onion for 4 to 5 minutes, then add the garlic, zucchini, yellow squash, bell pepper, and mushrooms. Cook for 8 to 10 minutes, stirring often, until the vegetables soften and most excess moisture has evaporated.

Stir in the squeezed spinach, Italian seasoning, kosher salt, and black pepper. Remove the skillet from the heat and let the vegetable mixture cool slightly while you prepare the cheese filling.

In a bowl, combine the ricotta, egg, Parmesan, and about 1 cup of the shredded mozzarella. Mix until evenly blended and creamy.

Spread a thin layer of marinara in the prepared baking dish. Add noodles, about one-third of the ricotta mixture, one-third of the vegetables, and more sauce, then repeat for three layers. Finish with noodles, sauce, and the remaining mozzarella.

Cover the dish tightly with foil and bake for 30 to 35 minutes. Remove the foil and continue baking for 15 to 20 minutes, until the sauce is bubbling and the cheese is lightly browned.

Let the lasagna stand for 15 minutes before cutting. Garnish with chopped basil or parsley if desired, then serve warm.