Whisk together the flour, 2 tablespoons sugar, and baking powder in a medium mixing bowl. Cut in the butter with a pastry blender until the mixture resembles coarse crumbs. Make a well in the center of the dry mixture; set aside.
Stir together the egg, half-and-half, and lemon peel in a small mixing bowl. Add the egg mixture all at once to the dry mixture; stir just until moistened.
Turn the dough out onto a lightly floured surface. Gently knead 10 to 12 times or until the dough is nearly smooth. Pat the dough into a 6-inch square. Cut the square into nine 2-inch squares with a floured knife; cut each square diagonally in half.
Place the triangles on an ungreased baking sheet. Brush the tops with a beaten egg white; lightly sprinkle with coarse sugar or granulated sugar.
Bake for 8 to 10 minutes or until golden. Remove the shortcake triangles from the baking sheet and cool on wire racks.
BERRY FILLING
Toss together the strawberries, raspberries, the ⅓ cup sugar, Grand Marnier and the 4 teaspoons snipped mint in large mixing bowl.
Cover and chill for up to 1 hour.
FINISHING
For each serving, spoon the berry mixture onto a dessert plate. Place three shortcake triangles on berry mixture.
Top with whipped cream and garnish with additional mint, if desired.