Prep the pan and oven: Heat the oven to 350°F (175°C).
Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy lifting.
Mix dry ingredients: In a large bowl, combine oats, almond flour, ground flaxseed, cinnamon, nutmeg, baking powder, and salt. Stir to distribute the spices evenly.
Whisk wet ingredients: In a separate bowl, whisk the egg, applesauce, maple syrup, milk, melted coconut oil or butter, and vanilla until smooth.
Combine: Pour the wet mixture into the dry ingredients. Stir until just combined.
The batter will be thick and scoopable.
Fold in add-ins: Gently fold in the diced apple, nuts, and raisins if using. Make sure the apple is evenly distributed for consistent moisture in every bite.
Spread and level: Transfer the batter to the lined pan. Use a spatula to press it into an even layer, reaching all corners.
Bake: Bake for 25–32 minutes, until the top is set, the edges are lightly golden, and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool completely: Let the bars cool in the pan for 20 minutes, then lift out using the parchment.
Cool fully on a rack before slicing. This helps the bars firm up.
Slice and serve: Cut into 9 larger bars or 12 smaller ones. Enjoy warm or at room temperature.