Preheat oven to 425°F.
Heat a cast-iron or oven-proof skillet over medium heat. (I like to grease my skillet with leftover bacon grease to keep the frittata from sticking to the sides of the pan at the end.) Remove sausage from casing and brown in skillet, breaking it up into little pieces as it cooks.
Meanwhile, whisk together the eggs, half of the cheese, and the salt and pepper in a medium bowl. Once the sausage is browned, pour the egg mixture over top and scrape the bottom of the pan to incorporate the sausage into the egg mixture. Continue to cook on the stove over medium heat for another 2-3 minutes until edges have set. While the egg mixture is cooking, arrange apple slices on top in a circular pattern, starting from the outside edge of the pan. Sprinkle the remaining cheese on top.
Transfer skillet to the oven. Bake for about 15-20 minutes, or until the frittata is set in the center and the cheese is golden brown on top. Remove from oven and cool slightly. Cut into wedges and serve. Enjoy!