Prepare the oven: Preheat the oven to 375°F (190°C). Lightly grease an 8 x 8-inch baking dish.
Mix the wet ingredients: Add 1 cup Greek yogurt, ½ cup unsweetened apple sauce, ¼ cup milk, ¼ cup honey, 1 large egg, and 1 teaspoon vanilla extract to a large bowl. Whisk until smooth and well combined.
Add the oats: Stir in 1½ cups rolled oats, 1½ teaspoons cinnamon, 1 teaspoon baking powder, and ¼ teaspoon salt. Mix until the oats are evenly coated.
Add apples and raisins: Gently stir in 2 medium apples (chopped) and ½ cup raisins, if using.
Fill the baking dish: Pour the mixture into the prepared dish and spread it out evenly. Sprinkle ½ cup walnuts (chopped) on top.
Bake the oatmeal: Bake at 375°F (190°C) for 35 to 40 minutes, until the center is set and the top is lightly golden. Let cool for 5 to 10 minutes before slicing and serving.