Preheat the oven to 190°C (375°F). Grease a 23-cm pie plate and sprinkle 1 tbsp of sugar over the bottom.
Arrange plum halves, cut-side down, to cover the entire bottom of the pie plate. Sprinkle 2 tbsp of sugar on top of plums.
To make the custard: Combine remaining 3 tbsp of sugar, eggs, milk, flour, lemon zest, cinnamon, vanilla, and salt in a blender. Blend until smooth, about 2 minutes. Pour over arranged plums in the pan.
Bake for 50 to 60 minutes until custard is firm and lightly browned. Let stand for 5 minutes before slicing. Dust with confectioners' sugar before serving.
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