01 - Preheat oven to 350°F (175°C). Grease a 9x9-inch baking dish with butter or nonstick spray.
02 - In a large mixing bowl, blend rolled oats, chopped nuts, baking powder, ground cinnamon, nutmeg, and salt until evenly distributed.
03 - In a separate bowl, whisk together milk, eggs, melted butter, maple syrup, and vanilla extract until smooth.
04 - Pour the wet mixture into the bowl of dry ingredients and stir until fully incorporated. Fold in diced apples and raisins or dried cranberries, ensuring even dispersion.
05 - Pour the combined mixture into the prepared baking dish and spread into an even layer.
06 - Place the dish in the oven and bake for 35 minutes, or until the top is golden brown and the center is set.
07 - Remove from oven and allow to cool for 5 minutes before serving. Serve warm, optionally with extra milk or yogurt.