Preheat oven to 375°F/190C and grease a medium (8 cup/2L capacity) baking dish.
In a large bowl, combine apple sauce, maple syrup, vanilla, cinnamon, and salt with a hand-held electric mixer until smooth.
Beat in the oat milk.
Stir in the chia seed and allow to sit for 5-10 minutes to thicken.
Peel and slice the apples into 1/4"/6mm thick slices.
Fold in the oats and baking powder until everything is wet and evenly distributed.
Fold in the walnuts and sliced apples.
Pour into the prepared baking dish and smooth it out evenly with a spatula.
Top with any reserved apple slices and walnuts.
Bake for 30-35 minutes until edges are just beginning to turn golden and the center is set.
Remove from oven an allow to cool for at least 10 minutes before serving.
Video
Notes
Watch the baking time. Keep an eye on the oatmeal in the last few minutes. You want golden edges but not too dark as you don’t want the oats to dry out.If possible, a narrower dish with higher sides is preferred for even moisture retention.Any milk will work in the recipe; it doesn’t have to be oat milk so long as you don’t need the recipe to be vegan.