Pre-heat oven to 350°F. Grease an 8x11" baking dish with butter or coconut oil and set aside.
In a medium bowl, combine all wet ingredients except the apple, slowly whisking in the melted butter last so it doesn't cook the eggs. Set aside.
In a large bowl, combine all dry ingredients and chopped apple. Add the wet ingredients and mix until just combined. Pour into prepared casserole dish and spread evenly. If desired, add apple slices on top.
Bake at 350°F for 40-45 minutes or until the top of the baked oatmeal is lightly golden and has some give but does not collapse easily when gently poked with a finger.
Allow to cool, then cut into 6 pieces for serving.
Notes
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