Prep Oats: Grind half of the oats in a blender or food processor until fine and flour-like (about 20-30 seconds).
Preheat Oven: Preheat oven to 1806C (3508F).
Mix Dry Ingredients: In a large bowl, combine ground oats, remaining whole oats, baking powder, salt, white sugar, and lemon zest.
1/2 cup rolled oats, 1/8 tsp salt, 1/4 tsp baking powder, 1 tsp lemon zest, 2 1/2 tsp white sugar
Mix Wet Ingredients: Add milk, vanilla extract (optional), yogurt, oil, and lemon juice. Stir until combined.
3 tbsp milk of choice, 1 tbsp lemon juice, 2 tbsp Greek yogurt, 1/2 tbsp neutral oil, 1/2 tsp vanilla
Add the Apple: Mix in the shredded apple.
1/2 small apple
Add the Apple: Lightly grease a baking dish or ramekin(s).
Fill: Pour batter evenly into the dish.
Bake: Bake for 30 minutes, turning off the oven for the last 5 minutes. Make sure the top is golden and a toothpick comes out clean.
Cool: Let cool slightly before serving.