Step 1Heat the oven to 350 degrees. Grease an 8-inch square baking pan. Line the bottom with parchment paper and spray the paper.
Step 2In a large bowl, combine the eggs and honey, whisking well to combine. Whisk in the milk and vanilla. Sprinkle over the baking powder, salt and cinnamon and whisk well to combine. Add the ground flaxseed, oats and apple and stir well to combine.
Step 3Transfer the mixture to the prepared pan, spreading it out with a rubber spatula and pressing down to create an even layer.
Step 4Bake until the oatmeal is firm to the touch and lightly colored on top, about 35 minutes. Cool the oatmeal for at least 15 minutes before transferring to a cutting board. Cut into 9 squares. Serve warm or at room temperature. Leftovers can be refrigerated in an airtight container up to 3 days, or wrapped well and frozen up to 3 months (defrost and warm in a toaster oven or 300-degree oven before eating).