Prep: preheat the oven to 350F. Peel your apples if desired (I did not), then chop them into 1/2" pieces.
Dry Ingredients: In a 9"x13" baking dish, add in the oats, flax, cinnamon, nutmeg, optional allspice/cardamom/ginger, baking powder, and salt. Whisk or mix well.
Wet Ingredients: Pour the non-dairy milk over the oats along with the maple syrup, and vanilla. Mix in the milk mixture well, then stir in the apples and walnuts, if using. Make sure the apples are spread evenly throughout the oat mixture.
Bake & Serve: bake in the middle rack of the oven for 35 minutes, depending on how thick you would like the oatmeal to be. For a “runnier” oatmeal, serve immediately after baking. Otherwise, let the oatmeal cool completely, then slice into bars. Serve as desired, warm or cold; leftovers will keep in the refrigerator for up to 5 days or can be pre-sliced and frozen for up to two months.