01 - Preheat oven to 350°F. Lightly grease a 9x9-inch baking dish with butter or non-stick spray.
02 - In a large mixing bowl, stir together the rolled oats, chopped nuts, brown sugar, baking powder, cinnamon, and salt until evenly distributed.
03 - In a separate bowl, whisk together the milk, eggs, melted butter, and vanilla extract until smooth and well combined.
04 - Pour the wet mixture over the dry ingredients and stir gently until the oats are fully coated and everything is evenly combined.
05 - Gently fold the chosen fruit into the batter, taking care not to overmix or crush delicate berries.
06 - Pour the batter into the prepared baking dish and spread it into an even layer using a spatula.
07 - Bake on the center rack for 35 to 40 minutes, until the top is golden brown and the center is set. A toothpick inserted in the middle should come out clean.
08 - Allow the baked oatmeal to cool for 5 to 10 minutes before slicing into squares. Serve warm with a splash of milk, a dollop of yogurt, or a drizzle of maple syrup.