Baked Apple Oatmeal

Name: Baked Apple Oatmeal
Ingredients: 4 tbsp. (1/2 stick) butter, melted, plus more, softened, for pan; 3 c. rolled oats (about 12 oz.); 1 tsp. baking powder; 0.5 tsp. ground cinnamon, plus more for sprinkling; 0.5 tsp. kosher salt; 0.125 tsp. freshly grated nutmeg; 2 large eggs; 3.5 c. whole milk; 0.25 c. honey, plus more for drizzling; 1 tsp. pure vanilla extract; 2 small tart apples (about 12 oz.; such as Granny Smith), cored, peeled, and chopped (about 1 3/4 c.); 0.5 c. chopped walnuts