Amish-Style Baked Oatmeal with Apples, Raisins & Walnuts

Name: Amish-Style Baked Oatmeal with Apples, Raisins & Walnuts
Prep time: 10 min
Cook time: 40 min
Total time: 50 min
Cuisine: American
Ingredients: 2 cups old-fashioned rolled oats (not instant); 3/4 cup light brown sugar; 1 cup chopped walnuts or pecans, divided; 1/2 cup raisins; 1 teaspoon baking powder; 2 teaspoons cinnamon; 1/2 teaspoon salt; 2 large eggs; 2 cups milk; 1 teaspoon vanilla extract; 4 tablespoons unsalted butter, melted, plus more for greasing the dish; 2 tart yet sweet baking apples, such as Honey Crisp, peeled and cut into 1/2-inch chunks (about 2 cups)