01 - Preheat oven to 350°F. Grease an 8x8-inch baking dish with cooking spray or butter.
02 - In a large bowl, whisk together the eggs, milk, melted butter, maple syrup, and vanilla extract until smooth and fully combined.
03 - Add oats, baking powder, cinnamon, nutmeg, and salt to the wet mixture. Stir until all ingredients are well incorporated and the oats are evenly coated.
04 - In a separate bowl, toss the diced apples with lemon juice, brown sugar, and cinnamon until the apples are evenly coated.
05 - Gently fold the apple mixture into the oat mixture, being careful not to overmix.
06 - Pour the mixture into the prepared baking dish and spread it evenly with a spatula.
07 - Bake for 35–40 minutes, or until the top is golden brown and the center is just set (a toothpick inserted should come out clean).
08 - Let the baked oatmeal cool for 5–10 minutes before serving to allow it to set.
09 - While the oatmeal cools, mix the Greek yogurt with 2 tbsp maple syrup in a small bowl until smooth and well combined.
10 - Cut the baked oatmeal into squares. Serve warm or at room temperature topped with a generous dollop of the sweetened Greek yogurt and a sprinkle of toasted nuts, if desired.