Step 1: Preheat your oven to 350°F (175°C). Generously grease a 9x13-inch baking pan with vegetable shortening or butter, then dust it lightly with flour. Set aside.
Step 2: In a medium bowl, combine the peeled, cored, and 1/4-inch sliced apples with 1/2 cup brown sugar and 1/2 teaspoon of ground cinnamon. Toss until the apples are well-coated. Set aside.
Step 3: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, remaining 1 tablespoon of ground cinnamon, baking powder, and salt.
Step 4: In a separate small bowl, whisk the eggs. Make a well in the center of the dry ingredients. Pour in the canola oil, orange juice, vanilla extract, and beaten eggs. Stir gently with a rubber scraper or wooden spoon until the mixture is just combined and no dry streaks remain. Be careful not to overmix.
Step 5: Spread approximately two-thirds of the cake batter evenly into the prepared baking pan. Arrange the cinnamon-coated apple slices over the batter, ensuring they are spread in a single layer and trying to keep them slightly away from the edges of the pan.
Step 6: Drop spoonfuls of the remaining cake batter over the apples, leaving some areas of the apples exposed. This creates a rustic, appealing look.
Step 7: Place the pan in the preheated oven and bake for 40-45 minutes, or until the top is golden brown and a toothpick inserted into a non-apple area of the cake comes out clean.
Step 8: While the cake cools slightly, prepare the icing. In a small bowl, whisk together the powdered sugar and 2 tablespoons of water until smooth. Add more water, a teaspoon at a time, if a thinner consistency is desired.
Step 9: Once the cake has cooled for about 15-20 minutes, drizzle the prepared icing evenly over the top.
Step 10: Slice and serve the Apple Cinnamon Sheet Cake warm for the best taste, either on its own or with a scoop of vanilla ice cream. Enjoy!