01 - Preheat oven to 350°F (175°C). Lightly grease a 9x9-inch (23x23 cm) baking dish with butter or nonstick spray.
02 - In a large mixing bowl, stir together rolled oats, chopped nuts, baking powder, cinnamon, nutmeg, and salt until uniformly mixed.
03 - In a separate bowl, whisk together milk, eggs, melted butter, maple syrup, and vanilla extract until well blended.
04 - Pour wet mixture into dry ingredients and mix until combined. Gently fold in diced apples and raisins or dried cranberries if using.
05 - Spread batter evenly in the prepared baking dish, ensuring fruit is distributed throughout.
06 - Place baking dish on center oven rack. Bake for 35 minutes, or until the top is golden and the mixture has set.
07 - Remove from oven and let rest for 5 minutes. Serve warm, with optional extra milk or yogurt.