Preheat the oven to 325°. Lightly butter a shallow 2-quart baking dish.
In a large bowl, whisk the whole eggs and egg whites with the granulated sugar, vanilla and lemon zest until pale, about 1 minute. Whisk in the cream. Sift the flour on top and whisk just until blended. Pour 1 cup of the custard into the baking dish and bake for 12 minutes, or just until set.
Scatter the raspberries over the custard and sprinkle with the superfine sugar. Pour the remaining custard on top and tamp down any floating raspberries. Bake for 50 minutes longer, until the custard is golden and set in the center. Let the custard cool slightly and serve warm or at room temperature.