Prep: Preheat the oven to 350°F. Grease an 8x8 inch baking dish (butter works best to prevent sticking).
Combine: In a medium mixing bowl, whisk together the maple syrup, eggs, milk, vanilla, brown sugar, salt, and melted butter until combined. Use a wooden spoon to stir in the oats, flaxseed, baking powder, cinnamon, peaches, and optional pecans. Mix until very well combined.
Bake: Spread the mixture out in the baking dish. Bake for 30-40 minutes for a metal pan or 35-45 minutes for glass/ceramic pan, until the middle is mostly set.
Vanilla Drizzle (optional): If you want more sweetness, stir together the milk, vanilla, pinch of salt, and powdered sugar in a small bowl. After the casserole cools some, drizzle this glaze over the top with a spoon.
Serve: Serve warm in bowls with milk poured over the top. (If you want it to taste more decadent, top with half and half or heavy cream.)