Preheat Oven – Preheat oven to 350°F (175°C). Grease a 9x9-inch baking dish with butter or non-stick spray.
Mix Dry Ingredients – In a large bowl, combine oats, cinnamon, baking powder, and salt. Stir well.
Mix Wet Ingredients – In another bowl, whisk together milk, eggs, maple syrup (or honey), vanilla, and melted butter until smooth.
Combine – Pour the wet mixture into the dry ingredients and stir until well combined.
Add Fruits & Nuts – Fold in apples, raisins, and walnuts.
Bake – Pour mixture into prepared baking dish and spread evenly. Bake for 35–40 minutes, or until the top is golden brown and the center is set.
Serve – Let it cool for about 10 minutes before slicing. Serve warm as is, or with a drizzle of maple syrup or a dollop of yogurt.