Mix the ingredients: Add the oats, milk, applesauce, walnuts, eggs, maple syrup, vanilla, cinnamon, nutmeg, baking powder and salt to a large bowl. Mix well.
Layer in a casserole dish: Lightly grease a 7×11 inch rectangular baking dish. Spread half the oat mixture in the bottom of the dish. Sprinkle with about ¾ of the peaches. Top with the remaining oat mixture, carefully smoothing the top. Sprinkle on the remaining peaches. You can also sprinkle on some extra walnuts and nutmeg if you like!
Chill the dish: Cover the casserole dish and chill in the refrigerator overnight (see notes for instructions to bake right away)
Bake the dish: The next morning, remove the dish from the fridge. Preheat the oven to 350°F. Bake until set and golden on top, about 30-35 minutes. Rest for 5 minutes on a rack on the counter, then slice and serve warm.
NOTE: To bake right away: Make sure to use quick-cooking oats, let the assembled oats sit on the counter for 10 minutes while you preheat the oven. UPDATE!***I use rolled oats and it's just fine***