01 - Preheat oven to 350°F. Lightly grease a 9x9-inch baking dish with butter or nonstick spray.
02 - In a large mixing bowl, combine rolled oats, chopped walnuts or pecans, baking powder, ground cinnamon, ground nutmeg, and salt. Stir until evenly distributed.
03 - In a separate bowl, whisk together milk, eggs, melted unsalted butter, maple syrup, and vanilla extract until smooth.
04 - Pour the wet mixture into the bowl with the dry ingredients. Stir gently until thoroughly combined.
05 - Fold in diced apples and raisins or dried cranberries, ensuring even distribution throughout the mixture.
06 - Pour the oatmeal mixture into the prepared baking dish and spread it out evenly. Bake in the oven for 35 minutes, or until the top is golden brown and the center is set.
07 - Let the baked oatmeal cool for 5 minutes before portioning and serving. Enjoy warm, optionally with extra milk or yogurt.