Preheat oven to 375°F/190C. Grease a medium (8 cup/2L capacity) baking dish. A narrower dish with higher sides is preferred for even moisture retention.

In a large bowl, combine apple sauce, maple syrup, vanilla, cinnamon, and salt. Use a hand-held electric mixer or a whisk to beat everything together 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 (optional: reserve some to sprinkle on the top)

Scrape into prepared baking dish.

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 and allow to cool for at least 10 minutes before serving.

Serve with additional fruit, a drizzle of maple syrup, or in a bowl with more oat milk.

Best the same day. Store covered in the fridge for up to 3 days. Reheat, covered, in the microwave or a 350°F/175C oven with a splash of water in the pan to restore moisture.