1. Preheat the oven to 350ºF.

2. Heat a 10-inch cast iron skillet over medium-high heat. Add the butter and melt, tilting the skillet, to coat it evenly on the bottom and sides.

3. Add the sausage to the skillet and cook, breaking up the meat, until browned and almost cooked through. Add the diced onion and cook, stirring, until softened, 3–5 minutes. Add the bell pepper and cook, stirring, for 2 minutes. Add the zucchini and garlic and cook, stirring, 2 minutes more.

4. While the sausage and veggies cook, in a large bowl, beat the eggs with the milk to thoroughly combine.

5. Once the veggies are cooked, reduce the heat under the skillet to low. Using a spatula, spread the sausage and veggie mixture into an even layer. Gently pour the eggs into the middle of the skillet and let cook until beginning to set, 1–2 minutes. Season the eggs with salt and pepper to taste.

6. Top the egg mixture with the Parmesan and shredded cheese. Continue cooking over low heat for about 5 minutes, then transfer the pan to the oven. Bake until the frittata is cooked all the way through, 5–10 minutes more.

7. Once cooked, carefully remove the frittata from the skillet, slice it into triangles, and serve with hot sauce and sour cream. Garnish with chives.