Preheat oven to 375 F.
Spread the zucchini out onto a baking sheet lined with paper towel or a kitchen towel and salt on both sides. Allow to sit for 15 minutes. Towel dry and press down the zucchini slices to remove the moisture.
In a nonstick skillet, heat the oil over medium high heat. Add in the sausage (out of the casing) and break it into small pieces. Once it's almost cooked through, add the onion and garlic. Cook for 2 minutes. Add in the mushrooms and cook for another 5 minutes until softened.
Add the tomatoes, basil, oregano and spinach. Simmer for about 10-15 minutes until the mixture is VERY thick and chunky. Season with salt and pepper, to taste.
In a small bowl, mix the ricotta, parm and egg.
Spread about 1/2 cup of sauce onto the bottom of a 9x13" baking dish. Add a layer of zucchini, then a layer of the ricotta mixture (about 1/2 cup) and a sprinkle of the grated havarti. Repeat with another layer of the sauce, zucchini, ricotta and havarti. You should have enough for three layers.
Cover the casserole dish with foil and bake for 35 minutes. Remove the foil and bake for another 15 minutes until golden brown. Allow it to stand for 5 minutes before serving.