Prepare the Vegetables. Preheat your oven to 375F. In a large skillet, heat olive oil over medium heat. Add the diced onion and minced garlic, and sauté until fragrant and translucent. Add the zucchini, yellow squash, eggplant, and mushrooms. Sauté for about 5 minutes, until the vegetables start to soften. Add the spinach leaves and cook until wilted. Season with salt, pepper, and dried Italian seasoning. Remove from heat and set aside.
Cook the Lasagna Noodles. Bring a large pot of salted water to a boil. Cook the lasagna noodles according to the package instructions until al dente. Drain and set aside. To prevent the noodles from sticking together, you can lay them out in a single layer on a sheet of parchment or wax paper.
Prepare the Tomato Sauce. In a bowl, combine the diced tomatoes and tomato paste. Season with salt, pepper, and dried Italian seasoning. Mix well to combine and set aside.
Prepare the Cheese Filling. In a separate bowl, combine the ricotta cheese, beaten egg, and grated Parmesan cheese. Mix well until thoroughly combined. Season with salt and pepper to taste.
Assemble the Vegetable Lasagna. In a greased 9x13-inch baking dish, spread a thin layer of tomato sauce on the bottom. Arrange three lasagna noodles on top of the sauce, slightly overlapping them. Spread a layer of the vegetable mixture on top of the noodles, followed by a layer of the ricotta cheese mixture. Repeat the layers until all the ingredients are used, finishing with a layer of tomato sauce. Sprinkle the shredded mozzarella cheese evenly over the top.
Bake and Garnish. Cover the baking dish with foil and bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 15-20 minutes, or until the cheese is golden and bubbly. Remove from the oven and let it cool for a few minutes before serving. Garnish with fresh basil or parsley for a pop of freshness and color.