Preheat your oven to 375°F (190°C) and lightly grease a baking dish to prevent sticking.
In a large mixing bowl, combine rolled oats, cinnamon, and chopped nuts if using, stirring to mix evenly.
In another bowl, whisk together milk, beaten egg, and maple syrup until smooth and well combined.
Pour the wet mixture into the dry ingredients and stir gently until everything is evenly coated and combined.
Fold in the diced apples and dried cranberries, distributing them evenly throughout the batter.
Pour the mixture into the prepared baking dish, spreading it out into an even layer.
Place the dish in the oven and bake for about 35 minutes, or until the edges are golden brown and the center is set with a slight wobble.
Remove the dish from the oven and let it cool slightly for a few minutes, allowing the texture to settle before serving.
Slice into squares and serve warm, topped with additional nuts or a drizzle of maple syrup if desired.