Preheat oven to 350 degrees F.
Cut off ends of zucchini and slice lengthwise, thinly (can use a mandoline slicer)
Spray baking pan with olive oil
Place zucchini sliced on sprayed baking pan and sprinkle with salt.
Bake in oven for 5 minutes, flip each piece and cook for an additional 5 minutes.
When done, set aside.
Heat olive oil in a large skillet.
Add onions, garlic, green pepper and mushrooms and saute until the onions are soft.
Add ground beef and brown completely.
Add spaghetti sauce and allow to simmer for 15-20 minutes.
In a large mixing bowl, combine ricotta, ½ cup of mozzarella, egg and drained spinach.
In an 8x8 baking dish, cover the bottom with a layer of meat sauce.
Layer on zucchini noodles (you may have to cut some noodles in half to fit)
Spread on layer of cheese mixture.
Repeat with another layer of meat mixture, noodles and cheese mixture.
Top with one last layer of meat mixture.
Cover dish with aluminum foil and bake at 350 degrees F for 30-35 minutes.
Remove from oven, remove foil and sprinkle on remaining ¼ cup of mozzarella.
Bake for an additional 10 minutes, until cheese is melted.