Preheat oven to 350°. Grease a 13"-by-9" dish with butter.
STEP 2
In a large bowl, whisk oats, baking powder, cinnamon, salt, and nutmeg.
STEP 3
In a medium bowl, whisk eggs, milk, honey, vanilla, and melted butter. Add to dry ingredients and mix until just combined. Stir in apples and walnuts. Transfer mixture to prepared dish.
STEP 4
Bake until oatmeal is set, golden brown, and apples are tender, 40 to 45 minutes. Let cool about 20 minutes. Serve warm with yogurt; drizzle with honey and sprinkle with cinnamon.