Preheat oven to 350 degrees F.
Make Topping: Add the following ingredients to a small mixing bowl- 3 Tablespoons rolled oats (old fashioned oats), 1 Tablespoon whole wheat flour, 1 Tablespoon dark brown sugar, 2 Tablespoons softened butter, ⅛ teaspoon cinnamon, pinch of salt. Cut together with fork or your hands.
Make Apple Filling: Add the following ingredients to your 7 ounce baking dish-1 teaspoon dark brown sugar, ½ teaspoon whole wheat flour, ⅛ teaspoon cinnamon, ⅛ teaspoon apple cider vinegar, and pinch of salt. Stir together.
Peel apple if desired then, chop into bite sized pieces, about 2 cups. Add to remaining apple filling ingredients. Toss until all apples are covered. If overflowing, remove some apples.
Spoon topping over apple filling. Bake for 25-30 minutes, until topping is golden brown and apples are bubbling.
Serve hot or at room temperature with ice cream, cream, or yogurt.