CRUST AND CRUMBLE
Preheat oven to 350Grease a 9-inch removable-bottom tart pan and place this on a baking tray.
In a medium bowl, whisk together flour, cornmeal, lemon zest, sugar, brown sugar, baking powder and salt. Add melted margarine (or butter). Using a wooden spoon, combine everything together forming large clumps.
Using floured hands, press two-thirds of the dough evenly into bottom and 1/2 inch up the sides of a prepared 9-inch tart pan with a removable bottom (you could also use a 9-inch cake pan); set aside the rest of the crust and crumble mixture.
In a small bowl, stir together the blueberries, brown sugar, vinegar, flour and salt. Spoon the filling into the crust, making sure you’re not touching the sides of the pan.
Crumble the remaining dough into large crumbs and scatter evenly over the filling, covering most of the surface.
Bake the tart for 40 - 45 minutes, until lightly browned and blueberry filling bubbles.
Leave to cool completely in the pan before slicing.