Spray an 8x8 or 2.5Qt oval casserole dish with cooking spray.
Directly in the casserole dish, mix together flaxseed, milk, brown sugar, and vanilla. Set aside while you peel and dice up the apples and banana.
To the casserole dish add oats, cinnamon, salt, baking powder, banana, apples, and pecans. Stir to mix and evenly distribute. Sprinkle 2 tsp of white sugar and ½ teaspoon of ground cinnamon over the top of oatmeal and place casserole in a 350-degree oven.
Bake until the top is golden brown and firm throughout, about 35 minutes. Serve with extra maple syrup and maybe milk for the top. Leftovers reheat nicely the next day.
The ground flaxseed helps thicken up the oats and is used as a replacement for an egg or two often found in baked oatmeal recipes. Let the flaxseed sit in the plant milk for a few minutes (while the oven warms up and you chopped the apples) so it has a chance to thicken up a bit before adding the remaining ingredients. For a firmer oatmeal casserole, use 2 tablespoons ground flaxseed instead of just one. Leftovers reheat nicely for breakfast the next day. My kids have been known to pack this into their lunches too.