In a 12 inch oven safe non stick skillet heat 2 tablespoons butter over medium heat. Add shallot and sauté until softened, about 3 to 5 minutes.
Add potatoes, cook about 5 minutes, until slightly browned.
In a bowl or large measuring cup, whisk 8 eggs with milk, salt and pepper. Be careful to mix just until incorporated. Over mixing will cause the frittata to poof in the oven. Mix in the mozzarella cheese. Set aside.
Preheat the oven to 350 degrees.
Back the stove. Add the last tablespoon of butter along with the zucchini, mushrooms, spinach and peppers. Sauté until the spinach wilts and everything is cooked through.
Add the eggs and rosemary. Mix together. Cook over medium heat until the egg starts to pull away from the pan. About 5 minutes.