Preheat your oven to 350°F (175°C) and lightly grease an 8×8-inch baking dish with butter or nonstick spray—this helps the bottom caramelize.

In a large bowl, whisk together oats, baking powder, cinnamon, nutmeg, and salt until evenly blended; if you have time, toast the dry mixture in a skillet over medium heat for 2–3 minutes, stirring, to coax out a nutty aroma.

In a separate bowl, whisk eggs, milk, melted butter, maple syrup, and vanilla extract; aim for a smooth, well-combined batter.

Pour the wet mix over the dry ingredients and stir until all oats are moistened; fold in the diced apples gently to avoid bruising the fruit.

Transfer the mixture to your prepared dish, smoothing the top with a spatula; tapping the pan on the counter helps settle everything. Bake for 35–40 minutes, or until the center is set and the top edges turn a light golden brown—you’ll see tiny oatmeal bubbles near the surface.

Remove from oven and let rest for 10 minutes; this pause lets the oats absorb any excess liquid and firms up the texture. Slice into six or eight squares and serve warm—top with extra milk, yogurt, or a spoonful of nut butter for richness.