1. Preheat the oven to 325'F
2. Grate the zucchini using a food processor or a box grate. Place the grated zucchini on top of a couple layers of paper towels and press to absorb moisture. Do this several times to ensure as much moisture is removed as possible.
3. In a 10 inch cast iron skillet (or oven proof skillet), melt the butter over medium heat. Add the onion, mushrooms, and zucchini and cook for 7-8 minutes. You should stir it occasionally to ensure it gets thoroughly cooked.
4. While the zucchini and onion mixture cooks, in another bowl whisk together the eggs, heavy cream, garlic, and black pepper.
5. Once the zucchini is cooked, remove it from heat and let it cool 5 minutes. Add it to the egg mixture with the mozzarella cheese and stir to combine thoroughly.
6. Pour the frittata mixture into the cast iron skillet (same pan you cooked the zucchini in), then place in the oven and bake for 25-30 minutes until set.
7. Remove from the oven and cut into 6 servings. The handle will be very hot for a while so be cautious when cutting the slices.