Preheat your oven to 350°F (175°C). Grease a 9-inch round baking dish or a similar-sized oven-safe dish.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt until well combined.
In another bowl, mix the milk, melted butter, eggs, lemon zest, and lemon juice. Pour this mixture into the dry ingredients and stir until just combined.
Pour the batter into the prepared baking dish. Carefully pour the hot water over the top of the batter; do not stir.
Bake for 30-35 minutes, or until the top is golden and a toothpick inserted into the center comes out clean.
Let it cool for a few minutes before serving. The pudding will have formed a delicious sauce underneath.