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, stir together rolled oats, chopped walnuts or pecans, baking powder, ground cinnamon, ground nutmeg, and salt until evenly blended.
03 - In a separate bowl, whisk together milk, large eggs, melted unsalted butter, maple syrup or honey, and vanilla extract until fully combined.
04 - Pour wet mixture into the bowl with dry ingredients and stir gently until just combined. Fold in diced apples and raisins or dried cranberries if using.
05 - Pour the oatmeal mixture into the prepared baking dish and spread evenly with a spatula.
06 - Bake in the center of the preheated oven for 35 minutes, or until the oatmeal is golden brown at the surface and set in the center.
07 - Let cool for 5 minutes before slicing and serving warm. Optional: Serve with extra milk or a spoonful of yogurt.