Preheat oven to 350 F. Line an 8-inch square baking pan with parchment paper.
In a small bowl, mix water and flax and let stand for a few minutes to thicken a bit.
In a medium bowl, mash bananas and stir in sugar and vanilla. Stir in the flax mixture.
In a larger bowl, mix the flour, oats, baking soda, baking powder and salt. Stir in the banana mixture, just until mixed.
Scoop about 2/3 of the batter into the prepared pan, and spread to an even layer. Sprinkle the blueberries over the batter, then dollop the remaining batter over the berries. Sprinkle the pecans evenly over the top.
Bake for 30 minutes, until firm and lightly golden. Let cool on a rack before slicing 3 by 4 to make 12 bars.