To make the filling, place the blueberries, 1/2 cup of the water, and brown sugar in a small, non-reactive saucepan. Bring to a boil and reduce heat to a simmer for a few minutes. Whisk the reserved 2 tablespoons water with the Clabber Girl Cornstarch and add to the simmering berries, stirring frequently for 2 to 3 minutes. Remove from the heat stir in the lemon zest and allow to cool.
Preheat the oven to 350 degrees F.
Mix together the oats, flour, brown sugar, Clabber Girl Baking Powder and salt. Using a wooden spoon, stir in the melted butter. Place half the mixture in the baking pan and pack it down firmly with your fingers. Bake for 15 minutes and cool briefly.
Pour the cooled blueberry mixture over the pre-baked crust and sprinkle the remaining half of the crumble crust over the top. Bake for 30 to 35 minutes, until the crust is a deep brown and the filling has bubbled through somewhat.
Recipe by Mani Niall - Mix it up with Mani