01 - Preheat oven to 350°F and grease an 8x8-inch baking dish with butter or cooking spray.
02 - In a large bowl, whisk together eggs, milk, melted butter, maple syrup, and vanilla extract until smooth and fully incorporated.
03 - Add oats, baking powder, cinnamon, nutmeg, and salt to the wet mixture. Stir until all ingredients are well combined and the oats are evenly coated.
04 - In a separate bowl, toss diced apples with lemon juice, brown sugar, and cinnamon until evenly coated.
05 - Gently fold the seasoned apple mixture into the oat mixture, ensuring even distribution throughout.
06 - Pour the mixture into the prepared baking dish and spread evenly with a spatula to create a uniform layer.
07 - Bake for 35 to 40 minutes until the top is golden brown and the center is just set. A knife inserted near the center should come out mostly clean.
08 - Let the baked oatmeal cool for 5 to 10 minutes before serving to allow it to set and make cutting easier.
09 - While the oatmeal cools, mix Greek yogurt with 2 tablespoons maple syrup in a small bowl until smooth and creamy.
10 - Cut baked oatmeal into squares and serve warm or at room temperature. Top each portion with a generous dollop of sweetened Greek yogurt and sprinkle with toasted nuts, if desired.