In a large bowl, mix flour, sugar and cornmeal; cut in butter until crumbly. Gradually add buttermilk, tossing with a fork until dough holds together when pressed. Shape into a disk; cover and refrigerate 30 minutes or overnight., Preheat oven to 375°. On a lightly floured surface, roll dough into a 14-in. circle. Transfer to a parchment-lined baking sheet., In a large bowl, combine berries, sugar, flour and lemon juice; spoon over crust to within 2 in. of edge. Fold crust edge over filling, leaving center uncovered. Brush folded crust with beaten egg; sprinkle with turbinado sugar., Bake 55-60 minutes or until crust is golden brown and filling is bubbly. Using parchment, slide tart onto a wire rack to cool. If desired, serve with whipped cream.