1. First, preheat your oven to 375°F (190°C). 
2. Using a mandoline slicer or a sharp knife, slice the zucchinis lengthwise into thin, even strips. 
3. Lay the zucchini strips on a baking sheet lined with paper towels. Lightly sprinkle them with salt and let them sit for about 15 minutes. 

Preparing the Filling

1. In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. 
2. Add the minced garlic and cook for another minute until fragrant.
3. Add the diced bell pepper and sliced mushrooms to the skillet.
4. Sauté the vegetables until they are tender, about 7–10 minutes. Add the chopped spinach last and cook until wilted. 
5. Season with salt, pepper, and Italian seasoning.

Preparing the Cheese Mixture

1. In a large mixing bowl, combine the ricotta cheese, 1 cup of shredded mozzarella, grated Parmesan, and the egg if you are using it.
2. For the vegan version, use vegan ricotta, vegan mozzarella, and nutritional yeast, and omit the egg.
3. Mix until well combined.

Assembling 

1. Spread a thin layer of marinara sauce on the bottom
2.  Layer zucchini strips over the sauce
3. Spread a layer of the cheese mixture over the zucchini. Then, add a layer of the sautéed vegetables.
4. Repeat these layers   until you have used all the ingredients.
5. Finish with the sauce and cheese on top
6. Repeat these layers — marinara sauce, zucchini strips, cheese mixture, and vegetables — until you have used all the ingredients.
7. After 30 minutes, remove the foil and bake for an additional 20–25 minutes.