Preheat oven to 350°. Grease the bottom and sides of a 13" x 9" glass cake pan with a touch of vegetable oil.
In a tiny bowl, mix ground flaxseed and water. Stir well, and set aside in the fridge to gel up.
In a large bowl, mix (dry ingredients) oats, brown sugar, salt, and baking powder. Whisk together well with a fork, breaking up clumps of brown sugar as you go. Sprinkle in extra cinnamon if desired.
In a medium bowl (or stand mixer), blend the eggs and the flaxseed mixture, then add oil, honey, and milk (wet ingredients). Whisk well until mixture is frothy, then add to the dry mix right away to maintain fluffiness. Stir together just until mixed well.
In a small bowl, melt butter for topping. Cream together the warm butter and brown sugar. Then, sprinkle on the cinnamon, flour, oats, and mix together. Topping should be crumbly.
Open the bag of still frozen blueberries and toss in 1 tbsp of flour. Zip the bag closed and toss blueberries until well coated. (This step is important so that your berries don't sink to the bottom.) Add these and (optional) chopped pecans to the wet mix, folding in gently.
Pour batter into greased cake pan. Sprinkle crumble topping over the top. Bake at 350° for 30-35 minutes. When done, it should come out a little jiggly, not totally dry.
Cool for at least 1 hour, then serve.