To prepare topping, weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, 1/4 cup granulated sugar, 2 tablespoons cornstarch, baking powder, and 1/8 teaspoon salt, stirring well. Cut butter into flour mixture with a pastry blender or two knives until mixture resembles coarse meal. Add half-and-half; gently knead dough just until moistened. Drop dough by spoonfuls evenly over top of filling. Combine almonds, turbinado sugar, and egg white; sprinkle over top. Bake at 350° for 50 minutes or until topping is browned. Let stand 10 minutes. Serve with ice cream.