1. Preheat the oven to 350°. Butter a 9-inch gratin dish. In a bowl, whisk the flour, sugar and a pinch of salt. Whisk in the eggs, butter and lemon zest until smooth. Add the milk and whisk until light and very smooth, about 3 minutes. Pour the batter into the gratin dish and top with the raspberries.
2. Bake for about 30 minutes, until the clafoutis is set and golden. Let cool slightly. Dust with confectioners' sugar, cut into wedges and serve.