Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish with 1 teaspoon butter.
Melt 1 tablespoon butter in a large skillet over medium heat. Add sausage, onion, and mushrooms; cook and stir until onion is translucent, about 5 minutes. Add spinach and garlic; cook and stir until spinach is wilted, about 2 minutes. Season with 1/2 teaspoon salt.
Combine remaining 7 tablespoons butter and water in a large saucepan; bring to a boil. Add potatoes and cook until fork-tender, about 10 minutes. Drain.
Whisk eggs, sour cream, and parsley together in a large bowl. Fold in sausage mixture, potatoes, and Cheddar cheese. Season with 1/2 teaspoon salt and black pepper. Turn out into the prepared baking dish.
Bake in the preheated oven until a knife inserted into the center comes out clean, about 20 minutes. Scatter goat cheese on top. Slice into 9 pieces.
