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 fully incorporated.
03 - Add oats, baking powder, cinnamon, nutmeg, and salt to the wet mixture. Stir until well combined and no dry spots remain.
04 - In a separate bowl, toss the diced apples with lemon juice, brown sugar, and cinnamon until 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 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 in the center should come out mostly clean.
08 - Let the baked oatmeal cool for 5–10 minutes before serving. This allows it to set and makes cutting easier.
09 - While the oatmeal cools, mix Greek yogurt with 2 tbsp maple syrup until smooth and well combined.
10 - Cut baked oatmeal into squares. Serve warm or at room temperature with a generous dollop of sweetened Greek yogurt and a sprinkle of toasted nuts, if desired.