Preheat oven to 350 degrees. In a large bowl, stir together rolled oats, brown sugar, nuts, baking powder, salt, and cinnamon. Reserve 1/4 cup of brown sugar for the topping.
In another bowl, whisk together milk, eggs, and vanilla. In a small microwave-safe bowl, melt butter in the microwave.
Add all ingredients together and place in an 8 or 9-inch baking dish. If you want to add fruit to the baked oatmeal, place it at the bottom of the baking dish. Some of the best fruits to use are -- apples, raspberries, strawberries, blackberries, etc. I would suggest using about 1 1/2 cups of fresh fruit.
Sprinkle remaining 1/4 cup of brown sugar all over the top of the oatmeal. This helps to create a sugary crust when it is baked. Bake for 35-45 minutes. Once it is baked, top with fresh berries or your favorite fruits. Serve warm.