Preheat the oven to 350°F (180°C).
Place the peaches in a large bowl. Using a zester or microplane, zest the lemon and add it to the peaches. Halve the lemon, squeeze its juice over the peaches, and toss to combine.
In a stand mixer fitted with the paddle attachment, cream together the butter and 1 cup (200g) of the sugar until light and fluffy. In a separate bowl, whisk together the flour, baking powder, and salt. Add the dry ingredients to the butter mixture in three batches, alternating with the milk, beginning and ending with the dry mixture.
Spray the inside of a 9x13-inch (23x33cm) baking dish with nonstick cooking spray. Pour the batter into the dish and spread it out evenly. Combine the remaining 1 cup (200g) of sugar and the hot water, stirring until the sugar dissolves. Pour the mixture over the batter; do not stir.
Bake for 35 to 40 minutes, until golden brown and set in the center. Let cool for 30 minutes, then serve warm with heavy cream.
