Preheat the oven to 350 degrees F.

Add the oil, spinach, and sliced zucchini to a 10-inch cast iron skillet and cover. Cook, stirring occasionally until spinach has wilted, about 2 minutes. Add the sun-dried tomatoes and a sprinkle of sea salt. Continue cooking, uncovered, until zucchini has some golden-brown color to it.

Beat the eggs and buttermilk, cheddar cheese, and sea salt in a mixing bowl. I do so with a fork, but you can also use a hand mixer or whisk.

Pour the egg mixture into the cast iron skillet over the vegetable mixture and use a wooden spoon to distribute the vegetables evenly. Sprinkle on the goat cheese.

Transfer the cast iron skillet to the oven and bake 25 to 30 minutes, or until the frittata has set up. Allow frittata to cool 5 minutes before slicing and serving.