Make the hawaij: In a small jar, stir together the cardamom, ginger, nutmeg, cloves, and cinnamon. Set aside.
Grease an 8-9 inch ovenproof dish with coconut oil. Preheat the oven to 375°F.
Assemble the oatmeal: Combine the ground flax seed and water in a small bowl. Stir to mix thoroughly, and then set aside to gel. Cut the cored apple into 1/4-inch thin slices. Then, cut each of those slices in half widthwise.
Heat the coconut oil in a medium sauté pan over medium heat. Add the apples to the pan. Let them sit for 2 minutes while they sizzle. Stir them around a bit, and sprinkle in 1/2 teaspoon of the hawaij spice mix. Stir to combine. Once the apples seem lightly softened on the outside, stir in 1 teaspoon of the maple syrup and the lemon juice. Remove the apples from the heat and allow them to cool.
In a large bowl, combine the rolled oats, baking powder, salt, and remaining 1/2 teaspoon of hawaij. Stir to combine. In a large measuring cup, whisk together the flax gel, non-dairy milk, and remaining 1/4 cup of maple syrup. Spread out half of the oat mixture in the oiled dish. Arrange half of the sautéed apples and half of the chopped nuts on top of the oats. Repeat these layers with the remaining oat mixture, apples, and nuts.
Slowly pour the milk mixture over the oats. Give the dish a tap to settle the contents. Dot the top of the oatmeal with little pieces of coconut oil if you like. Slide the oatmeal into the oven and bake for 35-45 minutes, or until the top of the oatmeal is golden brown and all of the liquid is absorbed.
While the oatmeal bakes, make the maple cream. In a medium bowl, vigorously whisk together the cashew butter, maple syrup, non-dairy milk, salt, vanilla, and lemon, if using. If the mixture seems too thick to drizzle, add more non-dairy milk by the teaspoon. Once totally smooth and pourable, you're good.
Serve the apple spice baked oatmeal hot with maple cream and extra maple syrup if you like.