How to Make It
Combine all of the ingredients except the oats in a food processor and pulse until coarse crumbs form. Transfer the mixture to a medium bowl and fold in the oats. Refrigerate until ready to use.
Preheat the oven to 350°. Grease a 9-by-13-inch cake pan. In a medium bowl, whisk the flour with the baking powder and salt. In a large bowl, combine the granulated sugar, butter and lemon zest and beat with a hand mixer at medium speed until light and fluffy, 3 to 5 minutes. Beat in the eggs one at a time, then beat in the buttermilk and vanilla; the mixture will look curdled. Add the dry ingredients and beat just until incorporated. Spread the batter evenly in the prepared pan, then spread a little of the batter up the sides of the pan all around. Spread the raspberries over the batter and top with the oatmeal streusel.
Bake the buckle for about 50 minutes, until the streusel is golden brown and a cake tester inserted in the center comes out clean. Let cool until warm.
Dust the buckle with confectioners' sugar and cut into squares. Serve warm or at room temperature.